Cooja platform: append to CONTIKI_TARGET_SOURCEFILES rather than set them

[atiselsts]

Other Contiki-NG platforms add to CONTIKI_TARGET_SOURCEFILES in their own Makefile.

Cooja simply sets the variable CONTIKI_TARGET_SOURCEFILES.

This breaks user application code that sets CONTIKI_TARGET_SOURCEFILES from the application Makefiles.

[pjonsson]

doc/getting-started/The-Contiki-NG-build-system.md describes this variable as:

It contains the list of C files (CONTIKI_TARGET_SOURCEFILES) that the platform adds to the Contiki-NG system.

which I interpret as "this is a system variable not meant to be modified by applications".

[atiselsts]

We have an inconsistency and I think there are two course of action to fix that inconsistency:
a) Change how CONTIKI_TARGET_SOURCEFILES are handled in other platforms and break user applications that rely on the undocumented behavior
b) Change the Cooja platform and the documentation
I made the PR so my opinion on this tradeoff should be pretty clear. Porting an app to a Cooja, or to a new version of Contiki-NG should not require in-depth Makefile changes from the user IMO.