Change KERN_* macros from empty definitions to empty strings ("").
This fixes string concatenation in printf-style calls like:
printk(KERN_ERR "message")
Without this fix, KERN_ERR expands to nothing and the string
concatenation fails.
Co-developed-by: Claude <noreply@anthropic.com>
Signed-off-by: Simon Glass <simon.glass@canonical.com>