Build: Fix MCUBOOT_KEY_S/NS export value

MCUBOOT_KEY_S/NS values vary in different cases and therefore export
their runtime values to NS, rather the default ones.

Change-Id: If0e5086ce8a47ac25d70f17ade18f2bf798c994f
Signed-off-by: David Hu <david.hu@arm.com>
diff --git a/cmake/install.cmake b/cmake/install.cmake
index 49f293b..d8e4bca 100644
--- a/cmake/install.cmake
+++ b/cmake/install.cmake
@@ -186,6 +186,9 @@
     install(FILES ${MCUBOOT_KEY_S}
             RENAME image_s_signing_private_key.pem
             DESTINATION ${INSTALL_IMAGE_SIGNING_DIR}/keys)
+    # Specify the MCUBOOT_KEY_S path for NS build
+    set(MCUBOOT_INSTALL_KEY_S
+        ${INSTALL_IMAGE_SIGNING_DIR}/keys/image_s_signing_private_key.pem)
     install(FILES $<TARGET_FILE_DIR:bl2>/image_s_signing_public_key.pem
             DESTINATION ${INSTALL_IMAGE_SIGNING_DIR}/keys)
 
@@ -194,7 +197,10 @@
                 DESTINATION ${INSTALL_IMAGE_SIGNING_DIR}/layout_files)
         install(FILES ${MCUBOOT_KEY_NS}
                 RENAME image_ns_signing_private_key.pem
-                DESTINATION ${INSTALL_IMAGE_SIGNING_DIR}/keys/)
+                DESTINATION ${INSTALL_IMAGE_SIGNING_DIR}/keys)
+        # Specify the MCUBOOT_KEY_NS path for NS build
+        set(MCUBOOT_INSTALL_KEY_NS
+            ${INSTALL_IMAGE_SIGNING_DIR}/keys/image_ns_signing_private_key.pem)
         install(FILES $<TARGET_FILE_DIR:bl2>/image_ns_signing_public_key.pem
                 DESTINATION ${INSTALL_IMAGE_SIGNING_DIR}/keys)
     endif()