PKG_CHECK_FORMAT_SECURITY: add a menuconfig option, disable by default

The idea is still to enable it by default at some point
I've tested all ar71xx packages (except oldpackages) using CONFIG_ALL=y
Failing packages have been marked with PKG_CHECK_FORMAT_SECURITY:=0 for now
I can test more targets but i have no idea which are the most used

Signed-off-by: Etienne CHAMPETIER <champetier.etienne@gmail.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@42282 3c298f89-4303-0410-b956-a3cf2f4a3e73