AP_Limits: fixed return of AP_Int8 parameters from object

we should return the base type, not the parameter, or callers could
use set on the result
build-PX4: changed the PX4 build to use CONFIG_C99_BOOL8

this fixes bool behaviour to match C99, which some of the APM code
assumed, including simple mode in ArduCopter
Rover: made a bit test clearer