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.
r264995 (nwhitehorn): Add script to setup bootable CD ISOs for both BIOS and EFI systems. Tested
and working on QEMU. Actually using this script as the regular image
generator, like with the memstick one, will require that the kernel support
EFI too. In particular, the following two things are required:
1. vt(9) be the default console driver
2. vt_efifb and vt_vga be able to coexist usefully in the same kernel

One other note here is that this requires newfs_msdos and mdconfig, which is
really ugly. NetBSD's makefs at least seems to support FAT now. If that
actually works, it should be imported and we can get rid of the mdconfig mess.
r265171 (nwhitehorn): Use mkimg instead of md(4) and gpart.
MFC EFI support for the installer

r264978 (nwhitehorn):

    Add EFI support to the installer. This requires that the kernel
    provide a sysctl to determine what firmware is in use. This sysctl
    does not exist yet, so the following blocks are in front of the
    wheels:
    - I've provisionally called this "hw.platform" after the equivalent
      thing on PPC
    - The logic to check the sysctl is short-circuited to always choose
      BIOS.  There's a comment in the top of the file about how to turn
      this off.

    If IA64 acquired a boot1.efifat-like thing (probably with very few
    modifications), the same code could be adapted there.

r265016 (nwhitehorn):

    Finish connecting up installer UEFI support. If the kernel was
    booted using EFI, set up the disks for an EFI system. If booted from
    BIOS/CSM, set up for BIOS.