Build: Add MBEDTLS_TARGET_PREFIX

Allows required targets to have prefixes added to them, so that external
projects can avoid target names clashing.

Signed-off-by: Raef Coles <raef.coles@arm.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 5af4c81..e4f318b 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -14,6 +14,10 @@
 #     CMake files. It is related to ZLIB support which is planned to be removed.
 #     When the support is removed, the associated include_directories command
 #     will be removed as well as this note.
+# - MBEDTLS_TARGET_PREFIX: CMake targets are designed to be alterable by calling
+#   CMake in order to avoid target name clashes, via the use of
+#   MBEDTLS_TARGET_PREFIX. The value of this variable is prefixed to the
+#   mbedtls, mbedx509, mbedcrypto and apidoc targets.
 #
 
 cmake_minimum_required(VERSION 2.6)
@@ -273,7 +277,7 @@
     add_subdirectory(programs)
 endif()
 
-ADD_CUSTOM_TARGET(apidoc
+ADD_CUSTOM_TARGET(${MBEDTLS_TARGET_PREFIX}apidoc
     COMMAND doxygen mbedtls.doxyfile
     WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/doxygen)