MFC r277215, r277291, r277418, r280953 and r280954:

loader: implement multiboot support for Xen Dom0

Note that only the subset of the multiboot specification needed in order to
boot a Xen Dom0 is implemented.

Sponsored by: Citrix Systems R&D

MFC 274398,274537:
Move NFS and TFTP filesystems before the synthetic filesystems (bzip,
gzip, and split).

MFC 274398,274537:
Move NFS and TFTP filesystems before the synthetic filesystems (bzip,
gzip, and split).