Add framework submodule with makefiles

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/.gitmodules b/.gitmodules
new file mode 100644
index 0000000..4fb26b5
--- /dev/null
+++ b/.gitmodules
@@ -0,0 +1,3 @@
+[submodule "framework"]
+	path = framework
+	url = https://github.com/Mbed-TLS/mbedtls-framework