buildman: Avoid hanging when the config changes

buildman: Avoid hanging when the config changes

Something has changed in the last several month such that when buildman
builds U-Boot incrementally and a new CONFIG option has been added to the
Kconfig, the build hanges waiting for input:

    Test new config (NEW_CONFIG) [N/y/?] (NEW)

Since binamn does not connect the build's stdin to anything this waits on
stdin to the build thread, which never comes. Eventually I suspect all the
threads end up in this state and the build does not progress.

Fix this by passing /dev/null as input to the build. That way, if there is
a new CONFIG, the build will stop (and fail):

    Test new config (NEW_CONFIG) [N/y/?] (NEW)

    Error in reading or end of file.

Signed-off-by: Simon Glass <sjg@chromium.org>