build: introduce STAGING_DIR_IMAGE

Introduce a new location STAGING_DIR_IMAGE which is intended to be used by
bootloader iamges and similar image-related artifacts.

This directory is guaranteed to be persistent across kernel upgrades which
might involve a removal of KERNEL_BUILD_DIR and is guranteed to be bundled
with the image builder.

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