Rename -image-arm64 to -image-uboot

There is nothing in the apertis-image-arm64 specific to the 64 bit arm
architecture. What the image is assuming is the usage of u-boot or
specifically a board running a bootloader (typically u-boot) that
supports u-boots distro boot configuration.

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Reviewed-by: Héctor Orón Martínez <hector.oron@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D7245