BUILD: Makefile: also report disabled options in the BUILD_OPTIONS variable

Now we iterate over all known variables and report in the BUILD_OPTIONS
string all those which differ from the target's defaults. This means that
if a target sets a variable by default (e.g. USE_THREAD in linux2628) and
the user disables it on the command line, the BUILD_OPTIONS string will
now properly report "USE_THREAD=".
1 file changed