[PXEXT] Change the PXE return code for EWOULDBLOCK

[PXEXT] Change the PXE return code for EWOULDBLOCK

Change the PXE return code for EWOULDBLOCK from PXENV_STATUS_FAILURE
to PXENV_STATUS_TFTP_OPEN.  This code is only used by the FILE_READ
PXEXT call, and is necessary to distinguish "error" from "no data" in
that call.

(The only other nonblocking call is UDP_READ, where the caller doesn't
care about the distinction, however, gPXE doesn't use EWOULDBLOCK
internally to represent this condition in that code.)