kill off the NuttX ADC driver config; we're not going to use it
First cut at a simple® ADC driver built on our driver framework.
Cleanup and add support for multiple channels.
Enable the temperature sensor channel for F2/F4 devices.
Configure ADC GPIOs on IO
Add trivial ADC support to PX4IO.
Add ADC measurements and reporting to PX4IO, including calibration for the battery input.
Introduced battery_status uORB topic, changed sensors app to publish to it, extended px4io driver to publish to it. Both do only so if the battery voltage is reasonably high, at 3.3V
Fixed stupid copy/paste typo
mavlink: fixed build warning
GPS: added comment on units for variance
build: cope better with absolute paths
build: allow for external apps via EXTERNAL_APPS

this is used by the APM build
Add FIONWRITE to allow applications to sniff the amount of writable space on a descriptor. Implement this for serial devices only.
serial: fixed up FIONREAD and FIONWRITE

the device ioctl returns -ENOTTY when it hasn't handled the command
added vdprintf() library function
added PWM_SERVO_SET_UPDATE_RATE ioctl

this allows apps to set the PWM update rate. APM needs this to honor
the user configurable PWM rate setting for copters.
fixup rate
fixed PWM_SERVO_GET

this was using the wrong channel
increase the UART buffer sizes to 256

The most critical one is the GPS serial port receive buffer size,
which needs to be at least 128 to support the UBLOX protocol, but it
seems a good idea for people running a FMU without a IO board to
increase the UART buffer sizes generally