Rename amd64 image to uefi

There is nothing specific in this image recipe to amd64, all it assumes
is that the board has a uefi first stage bootloader. This exists not
just for amd64, but also for for example 64 bit arm and 32 bit arm.

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/D7246