在 Linux 内核配置完成后，建立 ROOTFS 时，/dev 目录下的设备信息可以手动创建，但是也可以通过内核来自动产生。
自动产生 /dev 目录下设备的方法，是在内核配置过程中，加入对 DEVTMPFS 的选择。
> linux-4.10
CONFIG_DEVTMPFS:
This creates a tmpfs/ramfs filesystem instance early at bootup.
In this filesystem, the kernel driver core maintains device
nodes with their default names and permissions for all
registered devices with an assigned major/minor number.
Userspace can modify the filesystem content as needed, add
symlinks, and apply needed permissions.
It provides a fully functional /dev directory, where usually
udev runs on top, managing permissions and adding meaningful
symlinks.
In very limited environments, it may provide a sufficient
functional /dev without any further help. It also allows simple
rescue systems, and reliably handles dynamic major/minor numbers.
Notice: if CONFIG_TMPFS isn't enabled, the simpler ramfs
file system will be used instead.
Symbol: DEVTMPFS [=y]
Type : boolean
Prompt: Maintain a devtmpfs filesystem to mount at /dev
Location:
-> Device Drivers
-> Generic Driver Options
Defined at drivers/base/Kconfig:2
