Add an efi_warn logging helper for warnings, and implement an analog of
printk_once for once-only logging.
Signed-off-by: NArvind Sankar <nivedita@alum.mit.edu>
Link: https://lore.kernel.org/r/20200914213535.933454-1-nivedita@alum.mit.eduSigned-off-by: NArd Biesheuvel <ardb@kernel.org>