boot-directdisk/bootimg/image-vmdk: remove more nostamps for image generation

We have removed the nostamps for do_rootfs and do_build, so it seems
reasonable to remove the stamps for do_bootimg, do_bootdirectdisk and
do_vmdkimg.

The current problem is that the do_vmdkimg always re-run, but the
do_rootfs may not, so the code like below in the do_rootfs function
doesn't re-run and cause problems (the symlink exists when the
do_vmdkimg creates the symlink again):

    if d.getVar('IMAGE_LINK_NAME', True):
        cmds += "\trm -f ${DEPLOY_DIR_IMAGE}/${IMAGE_LINK_NAME}.*"

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>