AP_RPM: move PX4 IRQ handling into AP_HAL_PX4
AP_RPM: correct RPM sensor initialisation
The initialisation code used the type from the wrong configuration
parameters (if the first rpm sensor wasn't configured then the sensing
for the second sensor would use the type from the first).
The packing of drivers[...] was done in a non-sparse manner - i.e. if a
sensor wasn't detected then it would not take up space in the array.
The PX4 PWM backend relies on the instance number (offset in the drivers
array) corresponding to the parameters, so making this sparse is
required.
The main detection block fills in drivers based on the number of
instances detected so far, but the nullptr check checks based on the
number of detected backends. If the second instance wasn't configured we
wouldn't attempt to configure a third.
AP_RPM: add error reporting for attaching of interrupts
AP_RPM: use detach_interrupt method
AP_RPM: use (uint8_t)-1 in place of 255
Andrew Tridgell
Peter Barker