boot/syslinux: add an option to install a MBR blob

This MBR blob will look for an active partition, and boot the bootcode
present in that partition. This can be used to boot an extlinux-prepared
partition.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Lundquist <lists@zelow.no>
Cc: Frank Hunleth <fhunleth@troodon-software.com>
Tested-by: Frank Hunleth <fhunleth@troodon-software.com>
Tested-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>