kboot: Move common EFI stuff from aarch64 to libkboot

Move efi_read_from_pa, efi_set_systbl and efi_bi_loadsmap into efi.c
Move prototype for populate_avail_from_efi as well, though that arrived
previously.

Move efi memory map variables into efi.c.

Add efi_read_from_sysfs, but if 0 it out. We'll want to use it if we can
get the proposed /sys/firmware/efi/memmap data published in the same
format and the code works, it's just that the current
/sys/firmware/efi/runtime-memmap isn't complete enough to use.

Sponsored by: Netflix
Reviewed by: kevans, jhibbits
Differential Revision: https://reviews.freebsd.org/D49863