image.bbclass: remove tmp/rootfs after image creation unless you set a variable

image.bbclass: remove tmp/rootfs after image creation unless you set a variable

Set IMAGE_KEEPROOTFS to something other than the empty string to keep
tmp/rootfs around after image creation, otherwise it will be removed, as it's
not a fully functional filesystem, and causes much confusion among users as a
result.

Signed-off-by: Chris Larson <clarson@mvista.com>
Acked-by: Tom Rini <trini@embeddedalley.com>
Acked-by: Marcin Juszkiewicz <marcin@juszkiewicz.com.pl>