fix(build): race condition in sp layout generation

To prevent concurrency errors make it so generate_json.sh is
only called once during make. To do this supply a list of
present partitions to the script and generate the sp_layout
file using this.

Signed-off-by: Daniel Boulby <daniel.boulby@arm.com>
Change-Id: If9987cf5cd88be4ca617ce304e37d95346fb481b