dfu_mtd: Add provision to unlock mtd device

Prior to writing to an mtd device, mtd_erase is called. This call
fails in case the sector being erased is locked. Call mtd_unlock to
unlock the region which is to be erased and later written to. Lock the
region once the write to the region has completed.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>