mvebu: put u-boot images into image staging directory

Do not put the u-boot images into the kernel build directory as this directory
might get removed after kernel updates while the u-boot packages InstallDev
recipe is not getting re-executed because it is still considered current,
leading to image build failures later on due to missing u-boot images.

To ensure that built bootloader images persist over kernel version updates in
the buildroot, put them into the new STAGING_DIR_IMAGE directory.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>