pk no longer needs pk_write for ECDSA with MBEDTLS_USE_PSA_CRYPTO

The dependency is still useful for RSA, for which PSA encodes keys with an
ASN.1 structure.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

Author: gilles-peskine-arm

include/mbedtls/build_info.h

@@ -74,7 +74,10 @@
 #include MBEDTLS_USER_CONFIG_FILE
 #endif
 
-#if defined(MBEDTLS_PK_C) && defined(MBEDTLS_USE_PSA_CRYPTO)
+/* The PK wrappers need pk_write functions to format RSA key objects
+ * when they are dispatching to the PSA API. */
+#if defined(MBEDTLS_PK_C) && defined(MBEDTLS_USE_PSA_CRYPTO) && \
+    defined(MBEDTLS_RSA_C)
 #define MBEDTLS_PK_WRITE_C
 #endif