crypto: Add mbedtls-psa as a submodule

mbedtls-psa contains an implementation of libmbedcrypto, including the PSA
Crypto API.
diff --git a/tests/Makefile b/tests/Makefile
index b6e49bf..4118c14 100644
--- a/tests/Makefile
+++ b/tests/Makefile
@@ -12,15 +12,22 @@
 		-lmbedx509$(SHARED_SUFFIX)	\
 		-lmbedcrypto$(SHARED_SUFFIX)
 
+ifdef USE_CRYPTO_SUBMODULE
+LOCAL_LDFLAGS += -L../crypto/library
+CRYPTO := ../crypto/library/
+else
+CRYPTO := ../library/
+endif
+
 # Enable definition of various functions used throughout the testsuite
 # (gethostname, strdup, fileno...) even when compiling with -std=c99. Harmless
 # on non-POSIX platforms.
 LOCAL_CFLAGS += -D_POSIX_C_SOURCE=200809L
 
 ifndef SHARED
-DEP=../library/libmbedcrypto.a ../library/libmbedx509.a ../library/libmbedtls.a
+DEP=$(CRYPTO)libmbedcrypto.a ../library/libmbedx509.a ../library/libmbedtls.a
 else
-DEP=../library/libmbedcrypto.$(DLEXT) ../library/libmbedx509.$(DLEXT) ../library/libmbedtls.$(DLEXT)
+DEP=$(CRYPTO)libmbedcrypto.$(DLEXT) ../library/libmbedx509.$(DLEXT) ../library/libmbedtls.$(DLEXT)
 endif
 
 ifdef DEBUG