Build: Fix image signing if only S side is built

If there is no NS side to be built the S side signed binary added to
the make targets.
In addition in case of NS build if MCUBOOT_IMAGE_NUMBER is equal to 1
NS signed binary is not created, as it is not needed in such a
situation.

Change-Id: I1125402aaad273946f353ed71558aafa84a9a820
Signed-off-by: Mark Horvath <mark.horvath@arm.com>
1 file changed