build-sys: Add support for --enable-default-pcr-banks=list of PCR banks

Add an option that allows for the configuration of the default PCR bank
to use. This was currently hard coded to sha256 and now may be passed
via this option. The fallback is still sha256. Valid PCR bank names are
sha1, sha256, sha384, and sha512. The passed list must be a comma-
separated list of the valid PCR bank names.

Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
6 files changed