AP_HAL Semaphore: don't use limits, they don't work on c++
AP_HAL_AVR: add semaphore to I2CDriver
AP_HAL_Empty: add semaphore to I2CDriver
SITL: instansiate EmptyI2CDriver with EmptySemaphore
AP_GPS: fixed UBLOX example for PX4
AP_GPS: make UBLOX driver a bit more robust
Copter: enabled PX4 sensors
Copter: removed GPS_STATUS message

this message is huge, and completely pointless now that we have
num_satellites in GPS_RAW_INT
AP_Relay: fix pin used on APM2/SITL per Sando on drones-discuss

* My bad, thanks for the fix Sandro
AP_HAL_AVR: pull-up UART RX lines to fix GPS spoofing (Tridge)

Tridge discovered this bugfix:
https://groups.google.com/d/topic/drones-discuss/aek6LJeYQo8/discussion
HAL_PX4: support setting servo rate
HAL_PX4: update for new i2cdriver interface
AP_GPS: fixed debug code
MPU6000: fixed minor timing bug

if we miss a sample due to SPI contention we shouldn't update last
sample time
Copter: added main_loop_ready() function

this tells us when the main loop is ready to run. MAVLink won't send a
message if the main loop could run.
Copter: ensure we process deferred mavlink messages
Copter: a useful bit of timing debug code

enable when looking at main loop timing
Tools: use parallel builds in build_all.sh

saves developer time. Most devs have multi-core machines now
HAL_AVR: switch to maximum 256 byte buffers on UARTs

this makes the interrupt handler 8 bit, which saves enough cycles on
ArduCopter to be worthwhile.

We don't have any code that uses a buffer above 256 bytes.
DataFlash: fixed a semaphore bug in APM1 dataflash code