kboot: Method to populate the avail array from EFI map

If we can get an efi memory map, populate_avail_from_efi will create an
avail array. We only use the regiions marked as 'free' to find a place
for the kernel to land. The other regions are also eligible, but usually
too small to materially affect where we'd put the kernel (not to worry,
the kernel will use that memory).

Sponsored by: Netflix
Reviewed by: kevans, jhibbits
Differential Revision: https://reviews.freebsd.org/D49862