buildman: Add an option to ignore device-tree warnings

buildman: Add an option to ignore device-tree warnings

Unfortunately the plague of device-tree warnings has not lifted. These
warnings infiltrate almost every build, adding noise and confusion.

Add a buildman option to ignore them. This option works only with the
summary option (-s). It does not affect the build process.

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