boot - Add EFI support for placing /boot in a disklabel's 'a' partition.

boot - Add EFI support for placing /boot in a disklabel's 'a' partition.

* Add support to allow /boot to reside in the 'a' partition of a DragonFly
  disklabel64.  This allow /boot to be in either its own gpt partition or
  in the 'a' partition of a disklabel64 inside a gpt partition.

* Nominally allows the following gpt setup where all of DragonFly except
  the EFI System partition is contained in a DragonFly disklabel.

       2048     204800      0  GPT part - EFI System
     206848  124837888      1  GPT part - DragonFly Label64

  The disklabel can then be initialized as per the DragonFlyBSD standard,
  typically something like (example only):

    16 partitions:
    #          size     offset    fstype   fsuuid
      a:    1048576          0    4.2BSD    #    1024.000MB
      b:    8388608          *      swap    #    8192.000MB
      d:          *          *    HAMMER