Build: Add NS+S image signing to default build target

Add NS+S image signing targets to default build target.
Therefore, signing is always executed without specifying the specific
target name in build command.

Developer can leave the output target name empty in build command.

Change-Id: I117b296f59dacf88e3c9c0b2e3c2be2e8e6495e2
Signed-off-by: David Hu <david.hu@arm.com>
1 file changed