Improve makefile error message

This commit improves the makefile error message when using make, it no
longer incorrectly reports that CMakeLists.txt cannot be found instead
of exported.make.

Signed-off-by: Harry Ramsey <harry.ramsey@arm.com>
diff --git a/Makefile b/Makefile
index 3b89183..89142b1 100644
--- a/Makefile
+++ b/Makefile
@@ -8,13 +8,13 @@
         # GNU make appends ".  Stop.", so tweak the ending of our message accordingly.
         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.
+${MBEDTLS_FRAMEWORK_DIR}/exported.make not found (and does appear to be a git checkout). Run `git submodule update --init` from the source tree to fetch the submodule contents.
 This is a fatal error
-          endef
+            endef
         else
-          define error_message
-${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
+            define error_message
+${MBEDTLS_FRAMEWORK_DIR}/exported.make 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