stand: share bootinfo.c between EFI and KBOOT

Connect efi's bootinfo.c to the kboot build, and adjust to use
the kboot specific routines.

The getrootmount() call is independent of EFI. Remove ifdefs so it's
called for kboot too.

The differences between the kboot and efi bootinfo.c files are now tiny.
This could use some more refactoring, but this is a working checkpoint.

Sponsored by: Netflix
Reviewed by: tsoome
Differential Revision: https://reviews.freebsd.org/D38350