pw_kvs: Verify magic and checksum in flash
pw_kvs: Verify magic and checksum in flash

Change-Id: I9424379de2da47d294766e481e3d789a166766c3

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed