cmake: Extract common config to separate file

Move common parts from config cmake files to separate files.
Remove double slashes from paths.
Add basic sanity check to config cmake files.

Change-Id: I8e7512b2feba23250ddb7ad1340bb4ac5f75a206
Signed-off-by: Mate Toth-Pal <mate.toth-pal@arm.com>
diff --git a/secure_fw/spm/CMakeLists.inc b/secure_fw/spm/CMakeLists.inc
index 1808f66..015d635 100644
--- a/secure_fw/spm/CMakeLists.inc
+++ b/secure_fw/spm/CMakeLists.inc
@@ -1,5 +1,5 @@
 #-------------------------------------------------------------------------------
-# Copyright (c) 2017, Arm Limited. All rights reserved.
+# Copyright (c) 2017-2018, Arm Limited. All rights reserved.
 #
 # SPDX-License-Identifier: BSD-3-Clause
 #
@@ -40,7 +40,7 @@
 embedded_include_directories(PATH ${TFM_ROOT_DIR}/secure_fw/core ABSOLUTE)
 embedded_include_directories(PATH ${TFM_ROOT_DIR} ABSOLUTE)
 
-set(PLATFORM_DIR ${TFM_ROOT_DIR}/platform)
+set(PLATFORM_DIR ${TFM_ROOT_DIR}/platform/ext)
 embedded_include_directories(PATH "${PLATFORM_DIR}/cmsis" ABSOLUTE)
 embedded_include_directories(PATH "${PLATFORM_DIR}/target/sse_200_mps2/cmsis_core" ABSOLUTE)
-embedded_include_directories(PATH "${PLATFORM_DIR}//target/sse_200_mps2/sse_200/retarget" ABSOLUTE)
+embedded_include_directories(PATH "${PLATFORM_DIR}/target/sse_200_mps2/sse_200/retarget" ABSOLUTE)