rfkill: use update-alternatives

* Since oe-core added rfkill to busybox, this recipe needs to use u-a
  also.
* MJ: added ALERNATIVE_LINK_NAME[rfkill]
* MJ: increased ALTERNATIVE_PRIORITY (busybox also have 50)

Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>