tpm2_key_protector: Unseal key from a buffer

tpm2_key_protector: Unseal key from a buffer

Extract the logic to handle the file buffer from the SRK recover
function to prepare to load the sealed key from the NV index handle,
so the NV index mode can share the same code path in the later patch.
The SRK recover function now only reads the file and sends the file
buffer to the new function.

Besides this, to avoid introducing more options for the NV index mode,
the file format is detected automatically before unmarshaling the data,
so there is no need to use the command option to specify the file format
anymore. In other words, "-T" and "-k" are the same now.

Also update grub.text to address the change.

Signed-off-by: Gary Lin <glin@suse.com>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>