SPMD: generate and add Secure Partition blobs into FIP

Till now TF-A allows limited number of external images to be made part
of FIP. With SPM coming along, there may exist multiple SP packages
which need to be inserted into FIP. To achieve this we need a more
scalable approach to feed SP packages to FIP.

This patch introduces changes in build system to generate and add SP
packages into FIP based on information provided by platform.
Platform provides information in form of JSON which contains layout
description of available Secure Partitions.
JSON parser script is invoked by build system early on and generates
a makefile which updates FIP, SPTOOL and FDT arguments which will be
used by build system later on for final packaging.

"SP_LAYOUT_FILE" passed as a build argument and can be outside of TF-A
tree. This option will be used only when SPD=spmd.

For each SP, generated makefile will have following entries
     - FDT_SOURCES	+=	sp1.dts
     - SPTOOL_ARGS	+= 	-i sp1.img:sp1.dtb -o sp1.pkg
     - FIP_ARGS		+=	--blob uuid=XXXX-XXX...,file=SP1.pkg

Signed-off-by: Manish Pandey <manish.pandey2@arm.com>
Change-Id: Ib6a9c064400caa3cd825d9886008a3af67741af7