utils/diffconfig: remove BR2_* prefix restriction

utils/diffconfig: remove BR2_* prefix restriction

The utils/diffconfig script works only on variables with the BR2_
prefix. This is OK for Buildroot [def]configs since this is the prefix
for all user-facing variables, but it prevents using the same script
to compare configs from kconfig-based packages.

Remove the BR2_ restriction, allowing usage such as:

  ./utils/diffconfig \
	board/qemu/xtensa-lx60/linux.config \
	board/qemu/xtensa-lx60/linux-nommu.config

Signed-off-by: Marcel Patzlaff <m.patzlaff@pilz.de>
Reviewed-by: Luca Ceresoli <luca@lucaceresoli.net>
Tested-by: Luca Ceresoli <luca@lucaceresoli.net>
Reviewed-by: Matt Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>