RAUC: do not mark every build as -dirty

These per-package steps are (obviously) not being run with the external
tree as their working directory, which means that git would always
consider the working tree as "modified" -- hence the dirty status.

Fix this by informing git what I wanted to do in the first place.

Fixes: ca178a4b5b613d2ee62d631dadbbd6b8667a81ed ("RAUC: mark a modified git checkout appropriately in image metadata")
Change-Id: I5cec209d8446c950f0f59ebaf50e39ce0ef9fedf
diff --git a/package/czechlight-rauc/czechlight-rauc.mk b/package/czechlight-rauc/czechlight-rauc.mk
index 85d23a2..77161f6 100644
--- a/package/czechlight-rauc/czechlight-rauc.mk
+++ b/package/czechlight-rauc/czechlight-rauc.mk
@@ -5,7 +5,7 @@
 	$(RM) -rf $(CZECHLIGHT_RAUC_TMP_TARGET_DIR)
 	mkdir -p $(CZECHLIGHT_RAUC_TMP_TARGET_DIR)
 	sed \
-		-e 's|CZECHLIGHT_RAUC_IMAGE_VERSION|$(call qstrip,$(shell git --git-dir=$(BR2_EXTERNAL_CZECHLIGHT_PATH)/.git describe --dirty))|' \
+		-e 's|CZECHLIGHT_RAUC_IMAGE_VERSION|$(call qstrip,$(shell git --git-dir=$(BR2_EXTERNAL_CZECHLIGHT_PATH)/.git --work-tree=$(BR2_EXTERNAL_CZECHLIGHT_PATH) describe --dirty))|' \
 		-e 's|CZECHLIGHT_RAUC_COMPATIBLE|$(call qstrip,$(CZECHLIGHT_RAUC_COMPATIBLE))|' \
 		$(BR2_EXTERNAL_CZECHLIGHT_PATH)/package/czechlight-rauc/rauc-manifest.raucm.in \
 		> $(CZECHLIGHT_RAUC_TMP_TARGET_DIR)/manifest.raucm