lib: wrap crypt API to hide errno usage

In order to prevent using the global errno, replace it with a static
version and create a wrapper function which returns the error value.

Signed-off-by: Steffen Jaeckel <jaeckel-floss@eyet-services.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heiko Schocher <hs@denx.de>

include/crypt.h

@@ -9,5 +9,6 @@
  * @equal       Pointer to an int where the result is stored
  *                 '0' = unequal
  *                 '1' = equal
+ * @return 0 on success, error code of errno else
  */
-void crypt_compare(const char *should, const char *passphrase, int *equal);
+int crypt_compare(const char *should, const char *passphrase, int *equal);