Cooja: allow setting RSSI while sending and silence

It is now possible to set the RSSI while the radio is sending or when
no radiodata is received. This is now supported by all radiomediums
exept MRM. This also includes a GUI to configure the values.
Merge pull request #450 from darki2002/DGRM_support_BaseRSSI

Base RSSI support for DGRM