[build] Use a partition table in generated USB disk images

[build] Use a partition table in generated USB disk images

The USB disk image constructed by util/genfsimg is currently a raw FAT
filesystem, with no containing partition.  This makes it incompatible
with the use of CONSOLE_INT13, since there is no way to add a
dedicated log partition without a partition table.

Add a partition table when building a non-ISO image, using the mbr.bin
provided by syslinux (since we are already using syslinux to invoke
the ipxe.lkrn within the FAT filesystem).

The BIOS .usb targets are built using a manually constructed partition
table with C/H/S geometry x/64/32.  Match this geometry to minimise
the differences between genfsimg and non-genfsimg USB disk images.

Signed-off-by: Michael Brown <mcb30@ipxe.org>