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}.*"

(From OE-Core rev: 45c7dab5d51065c7b24ab5292f8e9d3104eb0626)

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

meta/classes/boot-directdisk.bbclass

@@ -97,4 +97,3 @@ python do_bootdirectdisk() {
 }
 
 addtask bootdirectdisk before do_build
-do_bootdirectdisk[nostamp] = "1"

meta/classes/bootimg.bbclass

@@ -198,4 +198,3 @@ python do_bootimg() {
 }
 
 addtask bootimg before do_build
-do_bootimg[nostamp] = "1"

meta/classes/image-vmdk.bbclass

@@ -28,7 +28,6 @@ python do_vmdkimg() {
 
 #addtask vmdkimg after do_bootimg before do_build
 addtask vmdkimg after do_bootdirectdisk before do_build
-do_vmdkimg[nostamp] = "1"
 
 do_vmdkimg[depends] += "qemu-native:do_populate_sysroot"