This can be either a single cpio archive with a .cpio suffix or a
space-separated list of directories and files for building the
initramfs image. A cpio archive should contain a filesystem archive
to be used as an initramfs image. Directories should contain a
filesystem layout to be included in the initramfs image. Files
should contain entries according to the format described by the
"usr/gen_init_cpio" program in the kernel tree.

When multiple directories and files are specified then the
initramfs image will be the aggregate of all of them.

See <file:Documentation/driver-api/early-userspace/early_userspace_support.rst> for more details.

If you are not sure, leave it blank.