build: Add top-level mbedtls_test target

In preparation of linking common test objects in programs,
add the top-level mbedtls_test target.

This target consists of the common test objects.

It is necessary to declare it at the top-level as both
tests and programs will depend on it and it is necessary
to synchronize the compilation of those objects for tests
and programs for the case of parallel building.

Signed-off-by: Ronald Cron <ronald.cron@arm.com>

tests/Makefile

diff --git a/tests/Makefile b/tests/Makefile
index 68f85d2..80c84fa 100644
--- a/tests/Makefile
+++ b/tests/Makefile
@@ -79,6 +79,8 @@
 
 MBEDTLS_TEST_OBJS=$(patsubst %.c,%.o,$(wildcard src/*.c))
 
+mbedtls_test: $(MBEDTLS_TEST_OBJS)
+
 # Rule to compile common test C files in src folder
 src/%.o : src/%.c
 	echo "  CC    $<"