doc: No longer mention Wicd in OS examples.

* gnu/system/examples/desktop.tmpl,
gnu/system/examples/lightweight-desktop.tmpl: Mention NetworkManager
instead of Wicd.

services: Missing services are automatically instantiated.

This simplifies OS configuration: users no longer need to be aware of
what a given service depends on.

See the discussion at
<https://lists.gnu.org/archive/html/guix-devel/2018-01/msg00114.html>.

* gnu/services.scm (missing-target-error): New procedure.
(service-back-edges): Use it.
(instantiate-missing-services): New procedure.
* gnu/system.scm (operating-system-services): Call
'instantiate-missing-services'.
* tests/services.scm ("instantiate-missing-services")
("instantiate-missing-services, no default value"): New tests.
* gnu/services/version-control.scm (cgit-service-type)[extensions]: Add
FCGIWRAP-SERVICE-TYPE.
* gnu/tests/version-control.scm (%cgit-os): Remove NGINX-SERVICE-TYPE
and FCGIWRAP-SERVICE-TYPE instances.
* doc/guix.texi (Log Rotation): Remove 'mcron-service-type' in example.
(Miscellaneous Services): Remove 'nginx-service-type' and
'fcgiwrap-service-type' in Cgit example.

tests: Cgit test waits for /var/run/shepherd/socket.

Previously tests sometimes start before the shepherd was listening,
leading to test failures.

* gnu/tests/version-control.scm (run-cgit-test)[test]: Add "shepherd
socket ready" test.  Rename to two "service running" tests for clarity.

services: shepherd: Make 'shepherd-configuration-file' non-monadic.

Suggested by atw on #guix.

* gnu/services/shepherd.scm (shepherd-service-file): Use 'scheme-file'
instead of 'gexp->file'.
(shepherd-configuration-file): Likewise, and adjust to non-monadic
style.
(shepherd-boot-gexp): Adjust accordingly.
* guix/scripts/system.scm (upgrade-shepherd-services): Use
'lower-object' in addition to 'shepherd-service-file'.

gnu: Add mkbootimg.

* gnu/packages/android.scm (mkbootimg): New variable.

gnu: Add abootimg.

* gnu/packages/android.scm (abootimg): New variable.

system: Add Nintendo NES Classic Edition installer.

* gnu/bootloader/u-boot.scm (u-boot-nintendo-nes-classic-edition-bootloader):
New exported variable.
* gnu/packages/bootloaders.scm (u-boot-nintendo-nes-classic-edition):
New exported variable.
* gnu/system/install.scm (nintendo-nes-classic-edition-installation-os):
New exported variable.

system: make-u-boot-package: Replace underscores by dashes in package name.

* gnu/packages/bootloaders.scm (make-u-boot-package): Replace underscores by
dashes in package name.

system: Factor out embedded-installation-os.

* gnu/system/install.scm (embedded-installation-os): New variable.
(beaglebone-black-installation-os): Use it.
(a20-olinuxino-lime2-emmc-installation-os): Use it.
(a20-olinuxino-micro-installation-os): Use it.
(banana-pi-m2-ultra-installation-os): Use it.

system: Factor out agetty-default-service.

* gnu/system/install.scm (agetty-default-service): New variable.
(beaglebone-black-installation-os): Use it.
(a20-olinuxino-lime2-emmc-installation-os): Use it.
(a20-olinuxino-micro-installation-os): Use it.
(banana-pi-m2-ultra-installation-os): Use it.

system: Add A20 OLinuXino MICRO installer.

* gnu/bootloader/u-boot.scm (u-boot-a20-olinuxino-micro-bootloader): New
exported variable.
* gnu/packages/bootloaders.scm (u-boot-a20-olinuxino-micro): New exported
variable.
* gnu/system/install.scm (a20-olinuxino-micro-installation-os): New exported
variable.

system: Add A20 OLinuXino LIME2 installer.

* gnu/bootloader/u-boot.scm (u-boot-a20-olinuxino-lime2-bootloader): New
exported variable.
* gnu/packages/bootloaders.scm (u-boot-a20-olinuxino-lime2): New exported
variable.
* gnu/system/install.scm (a20-olinuxino-lime2-emmc-installation-os): New exported
variable.

gnu: wxsvg: Update to 1.5.12.

* gnu/packages/wxwidgets.scm (wxsvg): Update to 1.5.12.

gnu: video, xml: Fix typo.

* gnu/packages/video.scm (libva)[synopsis]: Fix ‘libary’ typo.
[description]: Use @dfn.
* gnu/packages/xml.scm (libebml): Likewise.

gnu: godot: Fix building on aarch64-linux.

* gnu/packages/game-development.scm (godot)[arguments]: Add a flag when
building for aarch64-linux to build without threads.

gnu: cross-base: Apply gcc 6 patch to gcc >= 6.

* gnu/packages/cross-base.scm (cross-gcc): Apply gcc 6 patch to gcc >= 6.

gnu: u-boot: Update to 2018.01.

* gnu/packages/bootloaders.scm (define-module): Import (gnu packages gcc).
(u-boot): Update to 2018.01.
[native-inputs]: Add swig.  Remove dtc.
(make-u-boot-package): Use gcc-7.

system: Add Banana Pi M2 Ultra installer.

* gnu/bootloader/u-boot.scm (u-boot-banana-pi-m2-ultra-bootloader): New
exported variable.
(install-allwinner-u-boot): New variable.
(u-boot-allwinner-bootloader): New variable.
* gnu/packages/bootloaders.scm (u-boot-banana-pi-m2-ultra): New exported
variable.
* gnu/system/install.scm (banana-pi-m2-ultra-installation-os): New exported
variable.

gnu: graphicsmagick: Update to 1.3.28 [‘security fixes’].

* gnu/packages/imagemagick.scm (graphicsmagick): Update to 1.3.28.

gnu: deutex: Update to 5.1.1.

* gnu/packages/games.scm (deutex): Update to 5.1.1.