feat(build_library): reintroduce A/B menu.lst

reintroduce unique A/B menu.lsts to work around the kexec problems that
we have. Essentially instead of always using boot_kernel on pvgrub
systems use the A/B kernels installed at update time to the boot
partition.
Merge pull request #48 from philips/reintroduce-unique-a-b-menu.lst

feat(build_library): reintroduce A/B menu.lst