Makefile: allow overriding $(Q)

Allow Makefiles in Contiki-NG to
define a specific Q.

This enables Makefiles that include
Makefile.include to control the amount
of output, setting "Q = " shows everything
and a silent mode can be enabled
by setting "V = 1" and "Q = @".

Makefile.include

@@ -273,7 +273,7 @@ ifeq ($(V),1)
   TRACE_OBJDUMP  =
   TRACE_MKDIR =
   TRACE_CP =
-  Q=
+  Q ?=
 else
   TRACE_CC = @echo "  CC       " $<
   TRACE_CXX = @echo "  CXX      " $<
@@ -284,7 +284,7 @@ else
   TRACE_OBJDUMP  = @echo "  OBJDUMP  " $< "-->" $@
   TRACE_MKDIR = @echo "  MKDIR    " $@
   TRACE_CP       = @echo "  CP       " $< "-->" $@
-  Q=@
+  Q ?= @
 endif
 
 ### Forward comma-separated list of arbitrary defines to the compiler