yocto/conf: Refactor fvp-rd-kronos-specific configuration

Rename fvp-rd-kronos-extra.inc to
fvp-rd-kronos-cassini-extra-settings.inc to make use of Cassini's
existing support for including machine-specific settings.

The configuration in fvp-rd-kronos-virtualization.inc is not actually
machine-specific so move it to
conf/distro/include/arm-auto-solutions-virtualization.inc

Refactor how the AHCI image is attached to the FVP to avoid having a
machine-and-virtualization-specific conf file.

Signed-off-by: Peter Hoyes <peter.hoyes@arm.com>