kirkwood: support Linksys boot counter on EA[34]500

This is done with existing code from the WRT1900AC port.
It makes sure the "auto_recovery" bootloader option is set,
and resets the s_env boot counter after a successful boot.

This gives users without a serial console connection some
measure of safety.

Signed-off-by: Claudio Leite <leitec@staticky.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@47433 3c298f89-4303-0410-b956-a3cf2f4a3e73