altos: Allow AO_CONFIG_MAX_SIZE to be configured. Validate it.

TeleMega config is 200 bytes. AO_CONFIG_MAX_SIZE was 128. That didn't
work out well when logging erased flight information. Allow TeleMega
to use a larger value (1k), and then do a compiler hack to make sure
the defined value is at least as large as the ao_config structure.

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