Uniformize ifdefs to ECDSA_C+ECP_RESTARTABLE

Some parts were already implicitly using this as the two ifdefs were nested,
and some others didn't, which resulted in compile errors in some configs. This
fixes those errors and saves a bit of code+RAM that was previously wasted when
ECP_RESTARTABLE was defined but ECDSA_C wasn't
diff --git a/library/pk.c b/library/pk.c
index 70691d6..fae517a 100644
--- a/library/pk.c
+++ b/library/pk.c
@@ -73,7 +73,7 @@
     mbedtls_zeroize( ctx, sizeof( mbedtls_pk_context ) );
 }
 
-#if defined(MBEDTLS_ECP_RESTARTABLE)
+#if defined(MBEDTLS_ECDSA_C) && defined(MBEDTLS_ECP_RESTARTABLE)
 /*
  * Initialize a restart context
  */
@@ -99,7 +99,7 @@
     ctx->pk_info = NULL;
     ctx->rs_ctx = NULL;
 }
-#endif /* MBEDTLS_ECP_RESTARTABLE */
+#endif /* MBEDTLS_ECDSA_C && MBEDTLS_ECP_RESTARTABLE */
 
 /*
  * Get pk_info structure from type
@@ -203,7 +203,7 @@
     return( 0 );
 }
 
-#if defined(MBEDTLS_ECP_RESTARTABLE)
+#if defined(MBEDTLS_ECDSA_C) && defined(MBEDTLS_ECP_RESTARTABLE)
 /*
  * Helper to set up a restart context if needed
  */
@@ -225,7 +225,7 @@
 
     return( 0 );
 }
-#endif /* MBEDTLS_ECP_RESTARTABLE */
+#endif /* MBEDTLS_ECDSA_C && MBEDTLS_ECP_RESTARTABLE */
 
 /*
  * Verify a signature (restartable)
@@ -240,7 +240,7 @@
         pk_hashlen_helper( md_alg, &hash_len ) != 0 )
         return( MBEDTLS_ERR_PK_BAD_INPUT_DATA );
 
-#if defined(MBEDTLS_ECP_RESTARTABLE)
+#if defined(MBEDTLS_ECDSA_C) && defined(MBEDTLS_ECP_RESTARTABLE)
     if( rs_ctx != NULL && ctx->pk_info->verify_rs_func != NULL )
     {
         int ret;
@@ -256,9 +256,9 @@
 
         return( ret );
     }
-#else
+#else /* MBEDTLS_ECDSA_C && MBEDTLS_ECP_RESTARTABLE */
     (void) rs_ctx;
-#endif /* MBEDTLS_ECP_RESTARTABLE */
+#endif /* MBEDTLS_ECDSA_C && MBEDTLS_ECP_RESTARTABLE */
 
     if( ctx->pk_info->verify_func == NULL )
         return( MBEDTLS_ERR_PK_TYPE_MISMATCH );
@@ -350,7 +350,7 @@
         pk_hashlen_helper( md_alg, &hash_len ) != 0 )
         return( MBEDTLS_ERR_PK_BAD_INPUT_DATA );
 
-#if defined(MBEDTLS_ECP_RESTARTABLE)
+#if defined(MBEDTLS_ECDSA_C) && defined(MBEDTLS_ECP_RESTARTABLE)
     if( rs_ctx != NULL && ctx->pk_info->sign_rs_func != NULL )
     {
         int ret;
@@ -366,9 +366,9 @@
 
         return( ret );
     }
-#else
+#else /* MBEDTLS_ECDSA_C && MBEDTLS_ECP_RESTARTABLE */
     (void) rs_ctx;
-#endif /* MBEDTLS_ECP_RESTARTABLE */
+#endif /* MBEDTLS_ECDSA_C && MBEDTLS_ECP_RESTARTABLE */
 
     if( ctx->pk_info->sign_func == NULL )
         return( MBEDTLS_ERR_PK_TYPE_MISMATCH );