libconfuse: bump to version 2.7

Old libconfuse version 2.6 is from year 2007 so bump to version 2.7
Updated download URL in libconfuse.mk / Config.in
Removed libconfuse-2.6-dont-use-werror.patch
Changed := to just =

Signed-off-by: Dan Pattison <dan.pattison@ethertek.ca>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>