initramfs: add default_cpio_list, and delete -d option support

When CONFIG_INITRAMFS_SOURCE is empty, the Makefile passes the -d
option to gen_initramfs.sh to create the default initramfs, which
contains /dev, /dev/console, and /root.

This commit simplifies the default behavior; remove the -d option,
and add the default cpio list.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
diff --git a/usr/default_cpio_list b/usr/default_cpio_list
new file mode 100644
index 0000000..37b3864
--- /dev/null
+++ b/usr/default_cpio_list
@@ -0,0 +1,6 @@
+# SPDX-License-Identifier: GPL-2.0-only
+# This is a very simple, default initramfs
+
+dir /dev 0755 0 0
+nod /dev/console 0600 0 0 c 5 1
+dir /root 0700 0 0