image.bbclass: Ensure ${S} is cleaned at the start of rootfs generation

image.bbclass: Ensure ${S} is cleaned at the start of rootfs generation

Some image classes such as bootimg save files into ${S} as part of rootfs
generation. For correctness we should therefore clean this at the start of
image generation to ensure reproducibility.

I found this issue when some files I thought should disappear from my rootfs
would not disappear.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>