Build: Enable code sharing between bootloader and SPE

Add CMake functions to allow sharing regions of code between
independently linked binaries.

Signed-off-by: Tamas Ban <tamas.ban@arm.com>
Change-Id: I6a6132d6c1558b242d8da1dedab14f93a852f81a
diff --git a/config/config_default.cmake b/config/config_default.cmake
index c7fcde4..9f4d84d 100644
--- a/config/config_default.cmake
+++ b/config/config_default.cmake
@@ -28,6 +28,9 @@
 
 set(TFM_SPM_LOG_LEVEL                   2           CACHE STRING    "Set default SPM log level as INFO level")
 
+set(TFM_CODE_SHARING                    OFF         CACHE PATH      "Enable code sharing between MCUboot and secure firmware")
+set(TFM_CODE_SHARING_PATH               ""          CACHE PATH      "Path to repo which shares code with secure firmware")
+
 ########################## BL2 #################################################
 
 set(MCUBOOT_IMAGE_NUMBER                2           CACHE STRING    "Whether to combine S and NS into either 1 image, or sign each seperately")