Gpio: Refactor xpcc::Gpio::HIGH to xpcc::Gpio::High.
Gpio: Refactor xpcc::Gpio::LOW to xpcc::Gpio::Low.
I2C: Refactor xpcc::I2c::READ to xpcc::I2c::Read.
I2C: Refactor xpcc::I2c::WRITE to xpcc::I2c::Write.