Add missing clean target

* Throw away the tmp/ directory as recent OpenEmbedded is really good
with the sstate-cache and can rebuild it. This avoids having to try
to clean the sysroot or remove unused entries from work/

* Clean the sstate cache in case there are old entries.

Makefile

@@ -128,6 +128,10 @@ build.%: | git/poky
 %-upload: | build.$(subst -upload,,%) git/poky
 	$(Q)cd build.$(CUR_MACHINE) && ../scripts/upload-build.sh $(CUR_MACHINE) $(FEED_NAME)
 
+%-clean: | build.$(subst -clean,,%) git/poky
+	$(Q)cd build.$(CUR_MACHINE) && rm -rf tmp
+	$(Q)cd build.$(CUR_MACHINE) && ../git/poky/scripts/sstate-cache-management.sh  --cache-dir=sstate-cache -y -L --stamps-dir=tmp/stamps/
+
 # Create all build directories, build everything, upload everything, clean everything
 setup-all: | $(foreach machine, $(MACHINES), $(machine)-setup)