Revert "build: Skip rebuild of usr.img on osv.vmdk and osv.vdi target"

This reverts commit df30ec8d. It breaks
building "osv.vdi" altogether.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>

Makefile

@@ -44,7 +44,7 @@ check: export image ?= tests
 check: all
 	./scripts/test.py
 
-osv.vmdk osv.vdi: usr.img
+osv.vmdk osv.vdi: all
 	$(MAKE) -r -C $(dir $(submake)) $@
 .PHONY: osv.vmdk osv.vdi