treewide: sync bootcount scripts across targets

treewide: sync bootcount scripts across targets

This commit made the following changes to sync all bootcount scripts:

1. use boot() instead of start()
    This script only needs to be executed once when boot is complete.
    use boot() to make this explicit.

2. drop sourcing of /lib/functions.sh
    This is aready done in /etc/rc.common.

3. ramips: replace board name checking with a case

Signed-off-by: Chuanhong Guo <gch981213@gmail.com>