SPM: modify sptool to generate individual SP blobs

SPM: modify sptool to generate individual SP blobs

Currently sptool generates a single blob containing all the Secure
Partitions, with latest SPM implementation, it is desirable to have
individual blobs for each Secure Partition. It allows to leverage
packaging and parsing of SP on existing FIP framework. It also allows
SP packages coming from different sources.

This patch modifies sptool so that it takes number of SP payload pairs
as input and generates number of SP blobs instead of a single blob.

Each SP blob can optionally have its own header containing offsets and
sizes of different payloads along with a SP magic number and version.
It is also associated in FIP with a UUID, provided by SP owner.

Usage example:
sptool -i sp1.bin:sp1.dtb -o sp1.pkg -i sp2.bin:sp2.dtb -o sp2.pkg ...

Signed-off-by: Manish Pandey <manish.pandey2@arm.com>
Change-Id: Ie2db8e601fa1d4182d0a1d22e78e9533dce231bc