Create include/linux/kthread.h with stub definitions for kernel thread support. U-Boot doesn't have multi-threading. Includes: - kthread_create(), kthread_run(), kthread_stop() - kthread_should_stop(), kthread_should_park() - wake_up_process(), set_current_state() - task_pid_nr() Update compat.h to include kthread.h and remove duplicate definitions. Co-developed-by: Claude Opus 4.5 <noreply@anthropic.com> Signed-off-by: Simon Glass <simon.glass@canonical.com>