sys/boot/common: Make ufsread.c build and work with efi boot1 from FreeBSD.
* Bring ufsread.c a bit closer to the FreeBSD code (mostly by adding
fs_read_size(), which is needed for the efi boot1.
* So far ufsread.c was only used in sys/boot/pc32/boot2, where it was
built with -DBOOT2 in CFLAGS. Nothing should change there, since all
changes in this commit are either #ifndef BOOT2, or obviously shouldn't
make a difference for the pc32/boot2 (like adding the
boot2_ufs_read_size() function).
Partly-Taken-From: FreeBSD