Improve submodule error messages for Github archives

This commit improves the error messages informing users that have
downloaded Github archives to instead download a release archive. This
is due to Github not supporting submodules within archives and no
trivial way for users to use git to download them.

Signed-off-by: Harry Ramsey <harry.ramsey@arm.com>
diff --git a/Makefile b/Makefile
index e0a7510..3b89183 100644
--- a/Makefile
+++ b/Makefile
@@ -6,11 +6,16 @@
     ifeq (,$(wildcard framework/exported.make))
         # Use the define keyword to get a multi-line message.
         # GNU make appends ".  Stop.", so tweak the ending of our message accordingly.
-        define error_message
-$(MBEDTLS_PATH)/framework/exported.make not found.
-Run `git submodule update --init` to fetch the submodule contents.
+        ifeq (,$(wildcard .git))
+            define error_message
+${MBEDTLS_FRAMEWORK_DIR}/CMakeLists.txt not found (and does appear to be a git checkout). Run `git submodule update --init` from the source tree to fetch the submodule contents.
 This is a fatal error
+          endef
+        else
+          define error_message
+${MBEDTLS_FRAMEWORK_DIR}/CMakeLists.txt not found (and does not appear to be a git checkout). Please ensure you have downloaded the right archive from the release page on GitHub.
         endef
+        endif
         $(error $(error_message))
     endif
     include framework/exported.make