lib/libffi: Add libffi as a submodule.

This allows to build libffi from source together with micropython, and is
useful for cross-compilation. Support for this was already merged
previously, to use:

make libffi
make MICROPY_STANDALONE=1

(To both commands appropriate cross-compilition flags can be added).

.gitmodules

@@ -2,3 +2,6 @@
 	path = lib/axtls
 	url = https://github.com/pfalcon/axtls
 	branch = micropython
+[submodule "lib/libffi"]
+	path = lib/libffi
+	url = https://github.com/atgreen/libffi

libffi @ e9de7e35

+Subproject commit e9de7e35f2339598b16cbb375f9992643ed81209