fdt: avoid bad MAKEALL status

Current versions of  dtc  always print a message like

	DTC: dts->dtb  on file "dt.dtb.tmp"

which cannot even be suppressed with "-qqq".  To avoid incorrect
MAKEALL status, we manually filter out this message.  This is a bit
complicated, as we have to make sure to set a correct return code.

Also, get rid of the temp file: dtc accepts "-" for stdin.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Acked-by: Simon Glass <sjg@chromium.org>