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.
For reasons discussed long ago and that have to do with how ld handles weak symbols, it was decided that it was preferable to link using an explicit list of object files, instead of adding object files to an archive first and then linking using this archive. This was applied for the CC2538, but not for CC13xx/CC26xx.
This pull applies this link logic to all CM3s. For this to work with the Contiki-NG build system, we need `.SECONDEXPANSION:`, so this pull moves that to the common CM3 Makefile.
This commit fixes two bugs related to the usage of MODULES for CC2538-based platforms:
* newlib is no longer under `lib/newlib` but under `os/lib/newlib`.
* There is no such thing as `sys` under `arm/common`, so this module is removed.
