On Ubuntu/Debian, syslinux-common installs mbr.bin to
/usr/lib/syslinux/mbr/mbr.bin. This path is not currently searched by
find_syslinux_file(), causing USB disk image generation to fail with
"could not find mbr.bin".
Add /usr/lib/syslinux/mbr, /usr/share/syslinux/mbr, and
/usr/local/share/syslinux/mbr to the search paths.
Signed-off-by: Michael Brown <mcb30@ipxe.org>