Add installer support for PS3 and PowerNV systems, also laying the
foundation for invoking efibootmgr as part of new-style EFI booting on
x86. On PS3 and PowerNV, which are booted using Linux kexec from petitboot
rather than by loader(8), install the kernel and the rest of /boot to a
FAT partition and set up the appropriate petitboot configuration file
there.

The new bootconfig installer stage can do platform-dependent modifications
more complex than partition layout and installation of boot blocks and can
be used to (as here) set up some special configuration files, run efibootmgr,
or boot0cfg.

MFC after: 1 month