SCons: allow set object files in DefineGroup

The source files can be the raw file names end with .c or .o. So return
the raw file names is more flexible than wrap them with Object.
Merge pull request #376 from grissiom/tools-next

SCons: allow set object files in DefineGroup
efm32: return empty variable instead of Return('')
Delete trailing whitespace
Add mdk5 support in SCons tools
Add mdk5 project for stm32 bsps
Merge pull request #378 from grissiom/tools-next

efm32: return empty variable instead of Return('')
Merge pull request #381 from bright-pan/master

Add mdk5 building