altos: always rebuild ao_product.c to track git version

The git version is built into ao_product.c and saved in eeprom log
files, providing useful diagnostics about the firmware revision used
for each flight. However, if ao_product.c isn't recompiled, then the
updated version won't be included. Force recompilation of this file
each time make is run to ensure that the final output contains an
updated version number.

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