crypto: Add mbedtls-psa as a submodule

mbedtls-psa contains an implementation of libmbedcrypto, including the PSA
Crypto API.
diff --git a/Makefile b/Makefile
index f4c0a00..87b5a0c 100644
--- a/Makefile
+++ b/Makefile
@@ -28,7 +28,13 @@
 	mkdir -p $(DESTDIR)/lib
 	cp -RP library/libmbedtls.*    $(DESTDIR)/lib
 	cp -RP library/libmbedx509.*   $(DESTDIR)/lib
+ifdef USE_CRYPTO_SUBMODULE
+	mkdir -p $(DESTDIR)/include/psa
+	cp -rp crypto/include/psa $(DESTDIR)/include
+	cp -RP crypto/library/libmbedcrypto.* $(DESTDIR)/lib
+else
 	cp -RP library/libmbedcrypto.* $(DESTDIR)/lib
+endif
 
 	mkdir -p $(DESTDIR)/bin
 	for p in programs/*/* ; do              \
@@ -44,6 +50,9 @@
 	rm -f $(DESTDIR)/lib/libmbedtls.*
 	rm -f $(DESTDIR)/lib/libmbedx509.*
 	rm -f $(DESTDIR)/lib/libmbedcrypto.*
+ifdef USE_CRYPTO_SUBMODULE
+	$(MAKE) -C crypto uninstall
+endif
 
 	for p in programs/*/* ; do              \
 	    if [ -x $$p ] && [ ! -d $$p ] ;     \
@@ -85,6 +94,9 @@
 	$(MAKE) -C library clean
 	$(MAKE) -C programs clean
 	$(MAKE) -C tests clean
+ifdef USE_CRYPTO_SUBMODULE
+	$(MAKE) -C crypto clean
+endif
 ifndef WINDOWS
 	find . \( -name \*.gcno -o -name \*.gcda -o -name \*.info \) -exec rm {} +
 endif