at91bootstrap: put at91bootstrap images into image staging directory

at91bootstrap: put at91bootstrap images into image staging directory

Currently, building at91 subtargets via image builder will fail as the
required built at91bootstrap is not present in the image builder at all
since its presumed to be in the binary directory which obviously does not
exist yet.

So, lets install the at91bootstrap binary into image staging directory and
use that instead

Link: https://github.com/openwrt/openwrt/pull/18414
Signed-off-by: Robert Marko <robimarko@gmail.com>