feat(sptool): populate secure partition number in makefile

Calculate the secure partition number and saves it into the defined
macro NUM_SP.

Signed-off-by: Ben Horgan <ben.horgan@arm.com>
Signed-off-by: Leo Yan <leo.yan@arm.com>
Change-Id: I4175a10d315482b65fd0f3eed4c6fd1e1e2b5e4d
1 file changed