PK: reuse some eckey functions for ecdsa

Also add some forgotten 'static' while at it.
diff --git a/include/polarssl/ecp.h b/include/polarssl/ecp.h
index 2082bd9..36c6185 100644
--- a/include/polarssl/ecp.h
+++ b/include/polarssl/ecp.h
@@ -95,6 +95,8 @@
  * \brief           ECP key pair structure
  *
  * A generic key pair that could be used for ECDSA, fixed ECDH, etc.
+ *
+ * \note Members purposefully in the same order as struc ecdsa_context.
  */
 typedef struct
 {