FWU: Add FWU metadata access driver for MTD storage regions

In the FWU Multi Bank Update feature, the information about the
updatable images is stored as part of the metadata, on a separate
region. Add a driver for reading from and writing to the metadata
when the updatable images and the metadata are stored on a raw
MTD region.
The code is divided into core under drivers/fwu-mdata/ and some helper
functions clubbed together under lib/fwu_updates/

Signed-off-by: Masami Hiramatsu <masami.hiramatsu@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>