Build: Reorganize BL2 configuration in cmake

This commit moves the BL2 related configuration switches from the
CommonConfig.cmake file to a separate MCUBootConfig.cmake file to
make its maintenance easier.

Change-Id: I3869528bdbd2a5a071f3b3cc4b2782d22114b1ca
Signed-off-by: David Vincze <david.vincze@arm.com>
diff --git a/bl2/ext/mcuboot/MCUBoot.cmake b/bl2/ext/mcuboot/MCUBoot.cmake
index f685121..5187694 100644
--- a/bl2/ext/mcuboot/MCUBoot.cmake
+++ b/bl2/ext/mcuboot/MCUBoot.cmake
@@ -55,7 +55,7 @@
 		message(FATAL_ERROR "${MCUBOOT_SIGNATURE_TYPE} is not supported as firmware signing algorithm")
 	endif()
 
-	if (DEFINED SECURITY_COUNTER)
+	if (SECURITY_COUNTER)
 		set (ADD_SECURITY_COUNTER "-s ${SECURITY_COUNTER}")
 	else()
 		set (ADD_SECURITY_COUNTER "")