Build and test the fvp-rd-kronos image variants on CI.

Introduce linux-yocto-rt to enable the RT kernel to be tested with the
BSP in isolation.

Flip the EXTRA_IMAGE_FEATURES logic so that TESTIMAGE_AUTO is enabled
for arm-bsp-extras builds where no EXTRA_IMAGE_FEATURES are defined.

Add meta-arm-bsp-extras to yocto-layer-check.

Specify the KAS_MACHINE explicitly for all image build jobs.

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