Build: Improve dependency version handling

Add version variable for tf-m-tests, and update handling so that the
version variable always corresponds exactly to the git tag/hash/branch

Change-Id: Id4fe50487c599b12640704eb1bde79117c79db19
Signed-off-by: Raef Coles <raef.coles@arm.com>
diff --git a/lib/ext/mbedcrypto/CMakeLists.txt b/lib/ext/mbedcrypto/CMakeLists.txt
index 828a188..79adce1 100644
--- a/lib/ext/mbedcrypto/CMakeLists.txt
+++ b/lib/ext/mbedcrypto/CMakeLists.txt
@@ -20,7 +20,7 @@
     if (PATCH_FILES)
         FetchContent_Declare(mbedcrypto
             GIT_REPOSITORY https://github.com/ARMmbed/mbedtls.git
-            GIT_TAG mbedtls-${MBEDCRYPTO_VERSION}
+            GIT_TAG ${MBEDCRYPTO_VERSION}
             GIT_SHALLOW TRUE
             GIT_PROGRESS TRUE
             GIT_SUBMODULES ""
@@ -29,7 +29,7 @@
     else()
         FetchContent_Declare(mbedcrypto
             GIT_REPOSITORY https://github.com/ARMmbed/mbedtls.git
-            GIT_TAG mbedtls-${MBEDCRYPTO_VERSION}
+            GIT_TAG ${MBEDCRYPTO_VERSION}
             GIT_SHALLOW TRUE
             GIT_PROGRESS TRUE
             GIT_SUBMODULES ""