ci: Fix nightly to 2020-07-12

Fix nightly to a version where rustfmt was still available.

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

ci: Use atdf2svd from crates.io

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

ci: Test-build runtime feature

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

ci: Attempt building multple chips at once as well

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

Enable CI

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