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).
