Packages like binutils-*, busybox-static-*, gcc-*, grub-efi-*, musl-*
are only needed for the native architecture during cross compilation.
Don't bother with trying to build them for other arches to save time and
to avoid getting stuck frequently at "armv7/binutils-aarch64" etc.
A few people like to use pmbootstrap on aarch64 hosts (e.g. PineBook
Pro), so let's make it available for aarch64 again when we can build
aarch64 packages natively in CI and bpo. (They do get stuck there right
now, because of qemu user emulation.)
Related: https://gitlab.com/postmarketOS/build.postmarketos.org/-/issues/75
Use a deterministic mirror URL instead of CDN for aportgen. Otherwise we
may generate a pmaport that wraps an apk from Alpine (e.g. musl-armv7)
locally with one up-to-date mirror given by the CDN. But then the build
will fail if CDN picks an outdated mirror for CI or BPO.
While at it, also remove unnecessary "#!/usr/bin/env python3" in files
that only get imported, and adjust other empty/comment lines in the
beginnings of the files for consistency.
This makes files easier to read, and makes the pmbootstrap codebase more
consistent with the build.postmarketos.org codebase.
