BL2: Modify security_cnt.c to support higher MCUBOOT_IMAGE_NUMBER

Removes the dependency of code modification while updating
MCUBOOT_IMAGE_NUMBER in future. Resulted in removal of few
unused macros.

Signed-off-by: Arnold Gabriel Benedict <arnoldgabriel.benedict@arm.com>
Change-Id: I1645ef231143d304ca58a4535a216d27515870fa
5 files changed