With the exception of one specific build configuration for platform nrf52dk, all our arm-based platforms use the debugging I/O library from os/lib/dbg-io. This commit changes the build system to include this module by default for all arm devices. Platform / CPU Makefiles will no longer need to request this MODULE explicitly. Configuration provided to allow exclusion where required, as is the case for nrf52dk when NRF52_USE_RTT is set to 1.
