Use similar approach as r47461 to reduce output for busybox config.

Since r39435 CONFIG_BUSYBOX_CUSTOM=y has caused the addition of
all ~260 busybox config options to the diffconfig output even if
only one option has been changed.

This patch eliminates the unchanged busybox config options from the output
and leaves only the actually modified options.

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>

SVN-Revision: 47463

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

SVN-Revision: 47461

SVN-Revision: 36361

SVN-Revision: 26349

scripts/diffconfig.sh: use the new kconfig mode to remove some more unnecessary entries in the generated output

SVN-Revision: 25233

add scripts/diffconfig.sh: a tool for making reduced .config files these files contain only the target selection and options that differ from the default settings this makes it much easier to review .config files

SVN-Revision: 25231