Makefile: document the <pkg>-reinstall target

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
(cherry picked from commit 4899d9ec1b03370cbb4a8eecff3a6556dcc3e2e5)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

Makefile

@@ -1140,6 +1140,7 @@ help:
 	@echo '  <pkg>-dirclean         - Remove <pkg> build directory'
 	@echo '  <pkg>-reconfigure      - Restart the build from the configure step'
 	@echo '  <pkg>-rebuild          - Restart the build from the build step'
+	@echo '  <pkg>-reinstall        - Restart the build from the install step'
 	$(foreach p,$(HELP_PACKAGES), \
 		@echo $(sep) \
 		@echo '$($(p)_NAME):' $(sep) \