I think an Open Source project like this should support debugging through Open Hardware debug interfaces. The ones that come to mind are the FTDI based JTAG adapters and CoLinkEx ( http://www.coocox.org/Colinkex.htm ).
The EFM32 doesn't support JTAG, so FTDI based JTAG adapters aren't going to be of any help, unless they also happen to support SWD, which most don't. The CooCox looks interesting, but AFAICT it doesn't support any open source debugger.
The state of the art for open source SWD support is not very good yet.
EmBlocks supports JLink and STlink (or any other GDB server) both with GUI settings. In the install package is a STLink gdb server which supports flash/ram execution (settings of registers or memory like vector allocation table).
The Graphical user interface for the debugger probes are completely written with external scripts which can be modified or extended by users. So even OpenOCD or any other GDB can be supported with the same GUI mechanism. For the JLink there is currently ST, NXP and Generic UI interface scripts. It's quite easy to add EFM to the JLink product family with his own graphical UI instead of using the Generic's.
The JLink settings dialog for e.g. ST:
The install package contains all the necessary tools (special embedded GDB's and GCC's) to instantly start a project.