crypto: Add mbedtls-psa as a submodule

mbedtls-psa contains an implementation of libmbedcrypto, including the PSA
Crypto API.
diff --git a/programs/Makefile b/programs/Makefile
index b6d1fa2..d379ddf 100644
--- a/programs/Makefile
+++ b/programs/Makefile
@@ -14,6 +14,10 @@
 		-lmbedx509$(SHARED_SUFFIX)	\
 		-lmbedcrypto$(SHARED_SUFFIX)
 
+ifdef USE_CRYPTO_SUBMODULE
+LOCAL_LDFLAGS += -L../crypto/library
+endif
+
 ifndef SHARED
 DEP=../library/libmbedcrypto.a ../library/libmbedx509.a ../library/libmbedtls.a
 else