Add EFI ZFS boot support

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