android_mtdutils: Add mtd_read_descriptor

This CL adds "mtd_read_descriptor" to create a read context from a
file descriptor. It also refactors the code, especially
mtd_read_partition and read_block to alleviate the need of an
MtdPartition.

BUG=chromium:426742
TEST=emerge android_mtdutils

Change-Id: I1c976c271997b0c101b511c73081a70a7e0c1eaa
Reviewed-on: https://chromium-review.googlesource.com/234456
Reviewed-by: Nam Nguyen <namnguyen@chromium.org>
Commit-Queue: Nam Nguyen <namnguyen@chromium.org>
Trybot-Ready: Nam Nguyen <namnguyen@chromium.org>
Tested-by: Nam Nguyen <namnguyen@chromium.org>
2 files changed