fcb: start using errno codes

Switch form using privater FCB error codes to
errno codes. FCB private codes convention were compatible
with <errno.h> codes:
- 0 mean success
- negative values mean errors
- similar error types.
There was no sense to kept private FCB error codes.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>

include/fs/fcb.h

@@ -123,18 +123,6 @@ struct fcb {
 	 */
 };
 
-/*
- * Error codes.
- */
-#define FCB_OK		0
-#define FCB_ERR_ARGS	-1
-#define FCB_ERR_FLASH	-2
-#define FCB_ERR_NOVAR   -3
-#define FCB_ERR_NOSPACE	-4
-#define FCB_ERR_NOMEM	-5
-#define FCB_ERR_CRC	-6
-#define FCB_ERR_MAGIC   -7
-
 /**
  * @}
  */