Improved external interrupt handling

sbs.conf

src/shared/drivers/interrupt/external_interrupts.cpp

src/shared/drivers/interrupt/external_interrupts.h

src/shared/sensors/L3GD20.h

src/tests/drivers/test-l3gd20-fifo.cpp

src/tests/drivers/test-l3gd20.cpp

src/tests/test-interrupt.cpp