package/exfat(-utils): change license to GPL-2.0+

The license is specified in https://github.com/relan/exfat/blob/master/COPYING and indicates GPL-2.0+
The license changed from from GPL-3.0+ to GPL-2.0+ in 2013 but was never updated in buildroot.

https://github.com/relan/exfat/commit/48573fff5d070863e3279769e8a95d5c15a5c77d



Signed-off-by: Pieter Ronsijn <pieterronsijn@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>