make: make RIOT_VERSION fail-safe

Makefile.include

@@ -65,7 +65,7 @@ include $(RIOTBASE)/Makefile.cflags
 
 # make the RIOT version available to the program
 ifeq ($(origin RIOT_VERSION), undefined)
-  GIT_STRING := $(shell git describe --always --abbrev=4 --dirty=-`hostname`)
+  GIT_STRING := $(shell git describe --always --abbrev=4 --dirty=-`hostname` 2> /dev/null)
   ifneq (,$(GIT_STRING))
     GIT_BRANCH := $(shell git rev-parse --abbrev-ref HEAD)
     ifeq ($(strip $(GIT_BRANCH)),master)