armv8r64-extras/conf: Use bbclass to add machine-specific Zephyr config

The bbappend that is currently used to apply machine-specific Zephyr
configuration is problematic because it applies to all recipes with the
'zephyr-' prefix, including zephyr-sdk, which is not a Zephyr
application for the target.
Replace this mechanism with one using ZEPHYR_INHERIT_CLASSES, which is
only present in Zephyr application recipes. Create a bbclass,
zephyr-machine-extras, which includes a machine-specific .inc file, if
it exists.

Issue-Id: SCM-4874
Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>
Signed-off-by: Robbie Cao <robbie.cao@arm.com>
Change-Id: I3c0c3eb748aa192531299f573f611ceb65fb72cc