buildman: Detect Kconfig loops

buildman: Detect Kconfig loops

Hex and int Kconfig options are supposed to have defaults. This is so we
can configure U-Boot without having to enter particular values for the
items that don't have specific values in the board's defconfig file.

If this rule is not followed, then introducing a new Kconfig can produce
a loop like this:

   Break things (BREAK_ME) [] (NEW)
   Error in reading or end of file.

   Break things (BREAK_ME) [] (NEW)
   Error in reading or end of file.

The continues forever since buildman passes /dev/null to 'conf', and
the build system just tries again. Eventually there is so much output that
buildman runs out of memory.

We can detect this situation by looking for a symbol (like 'BREAK_ME')
which has no default (the '[]' above) and is marked as new. If this
appears multiple times in the output, we know something is wrong.

Add a filter function for the output which detects this situation. Allow
it to return True to terminate the process. Implement this termination in
cros_subprocess.

With this we get a nice message:

   buildman --board sandbox -T0
   Building current source for 1 boards (0 threads, 32 jobs per thread)
      sandbox:  w+   sandbox
   +.config:66:warning: symbol value '' invalid for BREAK_ME
   +
   +Error in reading or end of file.
   +make[3]: *** [scripts/kconfig/Makefile:75: syncconfig] Terminated
   +make[2]: *** [Makefile:569: syncconfig] Terminated
   +make: *** [Makefile:177: sub-make] Terminated
   +(** did you define an int/hex Kconfig with no default? **)

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