Fix CONTIKI_VERSION_STRING

Currently, the version string always ends with -dirty. This is related to the logic behind --git-dir, which assumes that the current working dir is the top-level dir of the repo, unless --work-tree is also specified

Makefile.include

@@ -263,7 +263,8 @@ CFLAGS += ${addprefix -I,$(SOURCEDIRS) $(CONTIKI)}
 ### Check for a git repo and pass version if found
 ### git.exe in Windows cmd shells may require no stderr redirection
 ifndef RELSTR
-RELSTR:=${shell git --git-dir ${CONTIKI}/.git describe --tags --always --dirty}
+RELSTR:=${shell git --git-dir ${CONTIKI}/.git --work-tree ${CONTIKI} describe \
+          --tags --always --dirty}
 endif
 
 ifneq ($(RELSTR),)