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.
Previously, the clean target would delete a number of file extensions, many of which currently unknown to our build system (old platforms?). This commit changes the target so that it only cleans what we know we are building. Platforms / CPUs can append to the target with platform-specific build artifacts.
