This builds on the modular EFI loader support added r294060 adding a
module to provide ZFS boot support on EFI systems.
It should be noted that EFI uses a fixed size memory block for all
allocations performed by the loader so it may be necessary to tune this
size.
For example when building an image which uses mfs_root e.g. mfsbsd, adding
the following to /etc/make.conf would be needed to prevent EFI from running
out of memory when loading the mfs_root image.
EFI_STAGING_SIZE=128
Submitted by: Eric McCorkle
MFC after: 2 weeks
X-MFC-With: r293268
Sponsored by: Multiplay
Make EFI boot loader modular in preparation for adding ZFS support.
This is a partial commit of the D4515.
Submitted by: Eric McCorkle
Reviewed by: emaste (in part)
MFC after: 2 weeks
X-MFC-With: r293268
Sponsored by: Multiplay
Differential Revision: https://reviews.freebsd.org/D4515
