buildman: Squash useless output from -K

When using #define CONFIG_SOME_OPTION, the value it set to '1'. When using
defconfig (i.e. CONFIG_SOME_OPTION=y) the value is set to 'y'. This results
in differences showing up with -K. These differences are seldom useful.

Adjust buildman to suppress these differences by default.

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