When WORKDIR is included in some other git checkout, version.sh calls git rev-parse
and it returns some description from that upper git checkout even when rfkill is
being built from release tarball.

When returned description doesn't match with expected v0.4, version.sh exits
without creating version.c
    # on git builds check that the version number above
    # is correct...
    [ "${descr%%-*}" = "v$VERSION" ] || exit 2

and build fails a bit later:
    | NOTE: make -j 32 -e MAKEFLAGS=
    |  CC   rfkill.o
    |  GEN  version.c
    | make: *** [version.c] Error 2
    | make: *** Waiting for unfinished jobs....
    | ERROR: oe_runmake failed

Don't try git rev-parse, if there isn't .git in ${S}.

--- a/version.sh	2013-11-15 03:43:12.587744366 -0800
+++ b/version.sh	2013-11-15 03:42:40.699743320 -0800
@@ -12,7 +12,7 @@
 
 if test "x$SUFFIX" != 'x'; then
 	v="$VERSION$SUFFIX"
-elif head=`git rev-parse --verify HEAD 2>/dev/null`; then
+elif test -d .git && head=`git rev-parse --verify HEAD 2>/dev/null`; then
 	git update-index --refresh --unmerged > /dev/null
 	descr=$(git describe 2>/dev/null || echo "v$VERSION")