README: Mention alternative location for pack downloads

The avr-rust organization has a mirror of all Atmel packs in a GitHub
repository.  Mention it as an alternative to the official download as it
is much easier to browse.

Closes #61.

Prepare 0.2.3

Signed-off-by: Rahix <rahix@rahix.de>

Update CHANGELOG

Add support for atmega328pb

fix: atmegaX8 TIFR should be read-write

fix: attiny84 ADCSRA should be read-write

README: Remove text about custom patching

We're now fully compatible with upstream svdtools so the additional
documentation is superfluous.  Remove it and point to the up-to-date
svdtools documentation instead.

Signed-off-by: Rahix <rahix@rahix.de>

Update CHANGELOG

Signed-off-by: Rahix <rahix@rahix.de>

README: Update dependencies

svdtools is needed now instead of PyYAML.

Signed-off-by: Rahix <rahix@rahix.de>

ci: Install svdtools

Signed-off-by: Rahix <rahix@rahix.de>

ci: Revert "Fix nightly to 2020-07-12"

This reverts commit 8c207c61f0611c6be152918828a17ab301e9387a.

Signed-off-by: Rahix <rahix@rahix.de>

patch: Switch to upstream svdtools patch tool

Instead of using our locally vendored version of svdpatch, switch to the
upstream version maintained by the stm32-rs project:

    https://github.com/stm32-rs/svdtools

Signed-off-by: Rahix <rahix@rahix.de>

patches: Include _svd tag

The _svd: tag is needed for upstream svdtools to find the corresponding
svd file for each patch.

Signed-off-by: Rahix <rahix@rahix.de>

Add basic support for ATmega644

Prepare 0.2.2

Signed-off-by: Rahix <rahix@rahix.de>

Update CHANGELOG

Simplify ATtiny84 ADC patch

Extract common EXINT patch from tiny84/85

Extract common EEPROM patch from tiny84/85/88

Extract common USI patch from tiny84/85