Merge pull request #21 from PX4/irqprio

Add "#ifdef CONFIG_ARCH_IRQPRIO" where needed
stm32_i2c: handle more error conditions on I2C bus

this fixes a I2C ISR storm condition where status=0x70084, which can
happen with some specific types of noise on the I2C bus. This was
reproduced by playing 80s rock through a Jabra wireless headset close
to the I2C cables.
Merge pull request #23 from tridge/pullrequest-i2c-lockup-fix

stm32_i2c: handle more error conditions on I2C bus
Merge branch 'master' of github.com:PX4/NuttX into usbdebug
Fix a linker error because of non-existent functions