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

1) Adding usr.img dependency to Makefile is broken because no usr.img
target is in Makefile. (It happened to work for me because I have a
file named usr.img under src/osv)

2) Adding usr.img dependency to build.mk will cause the rebuild of usr.img for some reason.

	osv.vmdk osv.vdi: usr.img
		...
	.PHONY: osv.vmdk osv.vdi

So, in order to build correct images. Let's drop the usr.img dependency
completely for now before we can fix issue 2).

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