image.bbclass: track ROOTFS_POSTUNINSTALL_COMMAND in do_rootfs vardeps

image.bbclass: track ROOTFS_POSTUNINSTALL_COMMAND in do_rootfs vardeps

The list of variables influencing do_rootfs was not updated when
introducing ROOTFS_POSTUNINSTALL_COMMAND. As a result, making changes
in commands listed there or the variables they depend on did not trigger
a re-run of do_rootfs.

(From OE-Core rev: 66b461ce9df7ed06d7651b9b54a49a950b97a1d4)

Signed-off-by: Patrick Ohly <patrick.ohly@intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>