Platform: Fix relative include paths of linker

Instead of using hard-coded relative include paths in the linker
command files, this change sets the include search path
in the command line.

Change-Id: I6d117a5bbf624f7c252f12103dee2afd338f2983
Signed-off-by: Gabor Kertesz <gabor.kertesz@arm.com>
diff --git a/platform/ext/Mps2AN521.cmake b/platform/ext/Mps2AN521.cmake
index 9c56ad4..1865d03 100755
--- a/platform/ext/Mps2AN521.cmake
+++ b/platform/ext/Mps2AN521.cmake
@@ -33,8 +33,9 @@
 else()
     message(FATAL_ERROR "No startup file is available for compiler '${CMAKE_C_COMPILER_ID}'.")
 endif()
-set (FLASH_LAYOUT          "${PLATFORM_DIR}/target/mps2/an521/partition/flash_layout.h")
-set (SIGN_BIN_SIZE         0x100000)
+set (FLASH_LAYOUT           "${PLATFORM_DIR}/target/mps2/an521/partition/flash_layout.h")
+set (PLATFORM_LINK_INCLUDES "${PLATFORM_DIR}/target/mps2/an521/partition/")
+set (SIGN_BIN_SIZE          0x100000)
 
 if (BL2)
   set (BL2_LINKER_CONFIG ${BL2_SCATTER_FILE_NAME})