yocto/[fvp-rd-kronos,linux]: Set virtual/kernel to linux-yocto-rt

Rename the bbappend file for linux-yocto to also apply to
linux-yocto-rt.

Add the configuration to fvp-rd-kronos-extras.inc to set the default
kernel to linux-yocto-rt.

Signed-off-by: Robbie Cao <robbie.cao@arm.com>