Standardize EFI's ESP mount point.

Mount the UEFI ESP on /boot/efi. No current system uses this by default, but
there are many ad-hoc schemes that do this in /efi or /esp or /uefi and adding a
new directory at the top-level would have a much higher likelihood of
collision. Document this in /etc/mtree/BSD.root.mtree and create EFIDIR and
related variables in bsd.own.mk.

Differential Revision: https://reviews.freebsd.org/D21344