cooja: add $(Q) to the customrules

This reduces the amount of output from
Cooja unless running with V=1.

Add an echo before the final link so
it's possible to diagnose whether the build
step completed when asking for Cooja output
during support requests. Example of error
message from a failing build:

Message:> make test-ringbufindex.cooja TARGET=cooja
Message:  CC        test-ringbufindex.c
...
Message:Redefining printf,sprintf,vsnprintf,etc.
Message:Linking build/cooja/mtype364.cooja
Message:/usr/bin/ld: i386 architecture of input file `build/cooja/mtype364.a(simEnvChange.o)' is incompatible with i386:x86-64 output
...