build(sptool): handle uuid field in SP layout file

Extract the UUID from the SP layout JSON file if the optional 'uuid'
field exists otherwise fall back to the current method for extracting
the SP UUID from the partition manifest file.

This change gives a way to decouple TF-A's dependency on the SP
manifest file's format which is tied to the SPMC.

Signed-off-by: Imre Kis <imre.kis@arm.com>
Change-Id: I13af066c1de58bfb9c3fd470ee137ea0275cd98c