Remap EBADF to PXENV_STATUS_TFTP_FILE_NOT_FOUND; the only user is

posix_io.c and this PXE error makes more sense in this context.

src/include/errno.h

 #define EALREADY			     ( PXENV_STATUS_UDP_OPEN | 0x0700 )
 
 /** Bad file descriptor */
-#define EBADF				   ( PXENV_STATUS_UDP_CLOSED | 0x0800 )
+#define EBADF				  ( PXENV_STATUS_TFTP_CLOSED | 0x0800 )
 
 /** Bad message */
 #define EBADMSG				      ( PXENV_STATUS_FAILURE | 0x0900 )