efi-pstore: Read and write to the 'compressed' flag of pstore

In pstore write, Efi will add a character 'C'(compressed) or
D'(decompressed) in its header while writing to persistent store.
In pstore read, read the header and update the 'compressed' flag
accordingly.

Signed-off-by: Aruna Balakrishnaiah <aruna@linux.vnet.ibm.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Tony Luck <tony.luck@intel.com>