Inserted a check into Resource.request() that controls if the current caller id has...

Inserted a check into Resource.request() that controls if the current caller id has a granted signal pending. This will prevent the caller to be inserted into the request queue which makes sense because the caller is the next one to be granted the resource.

This fixes a problem which prevented the RF230 driver from accessing the Spi bus when ActiveMessageAddress.changed() was signaled in the case of a ID change when using tosboot. This prevented the startup procedure to continue.