Build: Don't install signing scripts always

When PLATFORM_DEFAULT_IMAGE_SIGNING is disabled, don't install the image
signing scripts

Change-Id: I4d1b9f84e4703166327e8567f666c71c72131aab
Signed-off-by: Raef Coles <raef.coles@arm.com>
diff --git a/cmake/install.cmake b/cmake/install.cmake
index f0f6189..fbb9783 100644
--- a/cmake/install.cmake
+++ b/cmake/install.cmake
@@ -161,7 +161,7 @@
 
 ##################### Export image signing information #########################
 
-if(BL2)
+if(BL2 AND PLATFORM_DEFAULT_IMAGE_SIGNING)
     install(DIRECTORY bl2/ext/mcuboot/scripts
             DESTINATION ${INSTALL_IMAGE_SIGNING_DIR}
             PATTERN "scripts/*.py"