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>
158eed28be