Dynamically attach agtimer(4). Since agtimer(4) also provides the delay()

Dynamically attach agtimer(4).  Since agtimer(4) also provides the delay()
function for platforms that have it, rework the code a bit such that it can
be used before agtimer(4) attaches.  Introduce a new agtimer_init()
function that checks whether the CPU implements the Generic Timer feature
and switches to agtimer_delay() if that feature is present.  Call this
function from the generic platform initialization code.

ok jsg@