[ci:skip-build] CI fails because of an issue with pmb fetching build
deps on subsequent packages in the aggregated list of packages to build,
but bpo is expected to pass because it builds packages one at a time
These can be manually generated when one really wants to cross compile
from arm to x86_64. Usually this is not needed, so let's save the
compile time. [skip ci]
This covers most use cases and saves a lot of build time. Can be
changed on demand. Again, this simplifies package building as part of
the new build infrastructure effort. [skip ci]
Due to changes in abuild, our `gcc-armhf` etc. packages did not build
when using strict mode (i.e. `pmbootstrap build --strict gcc-armhf`)
anymore.
Changes:
* Set `CBUILDDIR=/`, so apk can read a valid package index from there
* Directly set `_cross_configure`, so it does not use CBUILDDIR anymore
* Set `BOOTSTRAP="nobuildbase"` to prevent apk from installing
`build-base-armhf` etc. (these don't exist in pmOS)
* Remove legacy code for lazy reproducible builds that wrapped
`package()`
Example: Building gcc-armhf for armhf does not make sense, so this
commit changes arch="all" to arch="aarch64 x86_64". This helps to
simplify the repository scripts (#970).
