MFC UEFI memstick build script

r264935 (gjb):

    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.

r264992 (nwhitehorn):

    Make a dual-boot BIOS/UEFI memstick image. Testing required before
    this gets renamed make-memstick.sh.

r265017 (nwhitehorn):

    loader's GPT support on BIOS does not seem to like the root
    filesystem being the last filesystem on the disk for some reason
    when made by this script. Add a vestigial swap partition to allow
    this to boot with QEMU BIOS.

Approved by:	re
Sponsored by:	The FreeBSD Foundation