image.bbclass: allow overriding dependency on virtual/kernel:do_deploy

image.bbclass: allow overriding dependency on virtual/kernel:do_deploy

Since the commit fe26b2379ecd ("image.bbclass: Depend on
virtual/kernel:do_deploy"), the image.bbclass made building images
depend on virtual/kernel. For some images, including small initramfs,
this is not the case. Allow overriding this dependency in case
developers knows what they are doing.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>