Some MMC cards support boot partitions. These are special regions
on the MMC card intended to put a bootloader on.

This patch adds support for these partitions, they are accessible
as /dev/diskx.boot[0|1].

Additionally the partitions can be configured bootable using a
device parameter. This can be used to mark the user area or one
of the boot partitions as bootable.

Since this feature is mostly seen on eMMC cards it is made optional
to lower the size impact for boards which do not have eMMC.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>