Add deprecation warnings

At this point in time, Mbed Crypto has no known security issues. But
in the future, it will not be updated if security issues are discovered.
So add warnings about the migration to Mbed TLS, but don't break the build yet.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/Makefile b/Makefile
index 4fd7f8e..c5b42a5 100644
--- a/Makefile
+++ b/Makefile
@@ -77,6 +77,10 @@
 	-scripts/config.py get MBEDTLS_TEST_NULL_ENTROPY && ([ $$? -eq 0 ]) && \
 	    echo '$(NULL_ENTROPY_WARNING)'
 endif
+	-echo "****************************************************************"
+	-echo "**** Mbed Crypto is no longer being updated. *******************"
+	-echo "**** Please use Mbed TLS instead. See README.md. ***************"
+	-echo "****************************************************************"
 
 clean:
 	$(MAKE) -C library clean