altos: Provide timeout value to ao_radio_recv

altos: Provide timeout value to ao_radio_recv

Instead of using ao_alarm around calls to ao_radio_recv, provide an
explicit timeout value as needed by radio functions with more
complicated system interaction than the cc1111. The timeout is 8 bits
of clock ticks.

Signed-off-by: Keith Packard <keithp@keithp.com>