kernel: add references to openwrt alternative button handling

Add documentation that makes it easier to find the alternative,
recommended, and default handling for buttons in OpenWrt.

Signed-off-by: Karl Palsson <karlp@tweak.net.au>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43989 3c298f89-4303-0410-b956-a3cf2f4a3e73