Previously, the accelerometer was only lazily initialised when it was
acccessed directly or an event handler was registered for
MICROBIT_ID_ACCELEROMETER. This patch also wakes the accelerometer on event
handlers registering for MICROBIT_ID_GESTURE.
Added support for automatic creation and assignment of the SERIAL_DEBUG symbol
when MICORBIT_DBG is enabled in MicroBitConfig.h
Also reordered the MicroBit constructor to bring up the serial object before
all other objects, to enable debugging of all code.