Add a separate script to build the memstick.img and the
mini-memstick.img with UEFI support.

As the comments in the file suggest, 1) there must
be existing ${.OBJDIR}/usr/src/release/{release,bootonly};
2) TARGET/TARGET_ARCH must be amd64; and 3) it must be
a vt(4)-enabled kernel with vt_efifb (*not* vt_vga).

This script is not hooked into release/Makefile in any way
until further testing is complete.

Sponsored by:	The FreeBSD Foundation