main/systemd-boot: new aport

EFI bootloader from systemd, with hacks to build it on Alpine/pmOS. Cross compilation (using a meson cross file) is used for building 32-bit version on x86_64, for systems that have a 32-bit EFI. Everything else assumes that the EFI arch matches the CPU arch.

Besides supporting all the archs we need, another major goal was to minimize the number of changes to systemd's build system required to build only the bootloader, so that maintaining/rebasing isn't too painful...

I am adding this to the "main" category, because I don't think there's a way to add it to Alpine. It requires cross compiling to x86 on x86_64 (to support 32-bit EFI on this arch), and Alpine doesn't support this. It requires stuff in pmaports/cross.

Note: if you want some back story on how I ended up submitting a MR to add systemd-boot to pmaports, see the full commit message!