Add MBEDTLS_FRAMEWORK_DIR variable to CMake

This commit adds a MBEDTLS_FRAMEWORK_DIR variable to CMake to create an
absolute path.

Signed-off-by: Harry Ramsey <harry.ramsey@arm.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 99e2169..b63b7c3 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -61,6 +61,7 @@
 
 # Set the project root directory.
 set(MBEDTLS_DIR ${CMAKE_CURRENT_SOURCE_DIR})
+set(MBEDTLS_FRAMEWORK_DIR ${CMAKE_CURRENT_SOURCE_DIR}/framework)
 
 option(ENABLE_PROGRAMS "Build Mbed TLS programs." ON)