altos: Do not block igniters on the radio mutex.

altos: Do not block igniters on the radio mutex.

Blocking igniters on the radio mutex fails when doing igniter testing
over the RF link; the packet receiver task will never release the
mutex and the CPU at the same time, causing the system to lock up.

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