Implement network boot support in efiboot(8). This changes efiboot(8)

Implement network boot support in efiboot(8).  This changes efiboot(8)
to recognize if it has been booted via PXE.  The new TFTP file system
layer will then use the matching PXE base code protocol handle to load
the files.  Since this uses the PXE base code protocol for abstraction
instead of the raw Simple Network protocol this will at this point not
work on u-boot based machines.

ok kettenis@