Error correction: Use libfec in blockimg.cpp for recovery

Add block_image_recover function to rewrite corrupted blocks on the
partition. This can be attempted if block_image_verify fails.

Note that we cannot use libfec during block_image_update as it may
overwrite blocks required for error correction. A separate recovery
pass in case the image is corrupted is the only viable option.

Bug: 21893453
Change-Id: I6ff25648fff68d5f50b41a601c95c509d1cc5bce