SST: Add ability to disable partial asset rw

- This patch adds compile time ability to disable partial asset RW
manipulation.
- Partial asset manipulation is allowed by default.

Change-Id: I9ddb527ee050cfe8fde2fccc9801025d6e5dfc71
Signed-off-by: Ben Davis <ben.davis@arm.com>
diff --git a/CommonConfig.cmake b/CommonConfig.cmake
index 6f7dab1..c0ac875 100755
--- a/CommonConfig.cmake
+++ b/CommonConfig.cmake
@@ -180,6 +180,10 @@
 	if (NOT DEFINED SST_VALIDATE_METADATA_FROM_FLASH)
 		set (SST_VALIDATE_METADATA_FROM_FLASH ON)
 	endif()
+
+	if (NOT DEFINED SST_ENABLE_PARTIAL_ASSET_RW)
+		set (SST_ENABLE_PARTIAL_ASSET_RW ON)
+	endif()
 endif()
 
 if (NOT DEFINED MBEDTLS_DEBUG)