altos: add ao_time_ns API
authorKeith Packard <keithp@keithp.com>
Mon, 25 Feb 2019 23:42:25 +0000 (16:42 -0700)
committerKeith Packard <keithp@keithp.com>
Mon, 25 Feb 2019 23:42:25 +0000 (16:42 -0700)
This provides nano-second resolution times by reading the systick
value (which runs at 250ns ticks on stm).

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

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