scripts/diffconfig.sh: reduce config output with CONFIG_BUSYBOX_CUSTOM

scripts/diffconfig.sh: reduce config output with CONFIG_BUSYBOX_CUSTOM

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>
Backport of r47463

git-svn-id: svn://svn.openwrt.org/openwrt/branches/chaos_calmer@47497 3c298f89-4303-0410-b956-a3cf2f4a3e73