altos: Move ao_storage_erase to shared code. Add len

altos: Move ao_storage_erase to shared code. Add len

This makes ao_storage_erase take an arbitrary length. The new version
erases each storage block if there's any un-erased data present, and
then also checks after erasing to make sure the block is clear.

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