image.bbclass: don't let do_rootfs depend on BUILDNAME
BUILDNAME is set by cooker as a string of current time. Letting do_rootfs
task depend on this variable gets us no benefit. Besides, letting do_rootfs
task depend on this variable will cause us trouble when executing
`bitbake -S none core-image-minimal'. With current code, this command
gives us error complaining about the different bashhash of do_rootfs task.
Signed-off-by: Chen Qi <Qi.Chen@windriver.com> Signed-off-by: Ross Burton <ross.burton@intel.com>
