HAL_AVR: make Console a direct wrapper of uartA

this saves a bunch of memory, and we don't really need separate
console support on AVR
Copter: move memcheck_init() earlier in setup
Plane: move memcheck_init() earlier in setup()
Copter: tweak the serial buffer sizes

we only need the large buffers for HIL
HAL_AVR: ensure we can handle a 256 size buffer