If you over allocate the number of analog channels this results in a
crash. It's easy to trigger this if you have voltage only monitors as we
still eat up a current input channel, regarless of if we use it. There
are only 16 channels at this time on ChibiOS, so if you have 9 voltage
only battery monitors you are out.
This PR improves that situation by only allocating channels when needed,
and in the case where we run out we now set a ConfigError, which on a
flight controller is much more friendly then a instant segfault the
moment we read a battery monitor. NOTE: on AP_Periph this takes the
node off the bus, rather then just sitting in the bootloader. This was
consideted acceptable as the current behaviour was to segfault and then
sit in the bootloader, unless you made new firmware that limited the
number of channels allocated it wasn't possible to recover in this
situation anyways.
