SPM: sptool: Introduce tool to package SP and RD ...

This tool packages Secure Partitions and Resource Descriptor blobs into
a simple file that can be loaded by SPM.

Change-Id: If3800064f30bdc3d7fc6a15ffbb3007ef632bcaa
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>