Add noreturn function attributes, add a comment.

Some of these attributes may be redundant but anyway..
FORCE_WATCHDOG=1 enables 4s watchdog timeout for the app.

This is to make sure the board is not bricked if it can't be reset
manually, after a user app locks up.

Also: in appStart() make sure we call the watchdog setup function before
setting r2 manually or it might get overwritten.
Merge branch 'master' of github.com:balrog-kun/optiboot
Update readme about FORCE_WATCHDOG.