board: samsung: exynos-mobile: use blkmap for booting from userdata subpartitions

board: samsung: exynos-mobile: use blkmap for booting from userdata subpartitions

Some distributions tend to provide a single combined image with EFS and
the system root filesystem. Flashing it as-is in a single partition
(usually done in userdata partition as it is the largest) is not
bootable as U-Boot does not understand subpartitions.

Use blkmap to map the userdata partition into its own block device.

Signed-off-by: Kaustabh Chakraborty <kauschluss@disroot.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>