distro: use -dirty marker on not clean builds

version string generated with 'git describe' contains -dirty marker if
not all changes have been commited before.

Signed-off-by: Patrick Zysset <patrick.zysset@netmodule.com>

classes/externalgitsrc.bbclass

@@ -7,7 +7,7 @@ def get_externalsrc_rev(d):
     import subprocess
     srcdir = d.expand(d.getVar("EXTERNALSRC"))
     #bb.note("external source: %s" % (srcdir))
-    rev = subprocess.check_output("cd %s && git describe --tag" % (srcdir),
+    rev = subprocess.check_output("cd %s && git describe --tags --dirty " % (srcdir),
                                   shell=True).strip()
     
     rev = rev.decode("utf-8")