ITS: Update filesystem and flash layer

Copies the filesystem files from SST and modifies them to rename the
symbols from ``sst`` to ``its`` and to update the implementation to be
aligned with the latest version of the PSA Storage specs.

Change-Id: If4e43b60be9e7fd3fbc161005c8947f252f4a457
Signed-off-by: Tudor Cretu <tudor.cretu@arm.com>
25 files changed