fastboot: add support for writing MBR

Add special target "mbr" (otherwise configurable via CONFIG_FASTBOOT_MBR_NAME)
to write MBR partition table.
Partitions are now searched using the generic function which finds any
partiiton by name. For MBR the partition names hda1, sda1, etc. are used.

Signed-off-by: Petr Kulhavy <brain@jikos.cz>
Reviewed-by: Tom Rini <trini@konsulko.com>
Acked-by: Steve Rae <steve.rae@raedomain.com>
Reviewed-by: Simon Glass <sjg@chromium.org>