yocto/zephyr-kernel: Remove references to Kronos

To better generalize the Zephyr app configuration, substitute
MACHINE into ZEPHYR_BOARD. meta-zephyr post-processes this
variable and creates BOARD, which is a valid Zephyr board name.

Replace all references to ZEPHYR_BOARD with BOARD.

Remove references to Kronos from the patch files and variables.

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