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.

r268256 (nwhitehorn):

    After EFI support was added to the installer, it needed to allow
    boot partitions of types other than "freebsd-boot" (in particular,
    "efi").  This allows the removal of some nasty hacks for supporting
    PowerPC systems, in particular aliasing freebsd-boot to apple-boot
    on APM and an IBM-specific code on MBR.

    This changes the installer to use the correct names, which also
    breaks a degeneracy in the meaning of "freebsd-boot" that allows the
    addition of support for some newer IBM systems that can boot from
    GPT in addition to MBR. Since I have no idea how to detect which
    those systems are, leave the default on IBM PPC systems as MBR for
    now.

Approved by:	re
PR:		193658
Relnotes:	Yes
MFC r268540 by nwhitehorn:

    Allow up to 512K for boot code on GPT with BIOS. As pmbr.s says in
    a comment, "545K should be enough for any boot code". This rounds
    down slightly.

Approved by:	re