at91: put u-boot images into image staging directory

at91: put u-boot images into image staging directory

Currently, building at91 subtargets via image builder will fail as the
required built u-boot 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 u-boot 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>