CONFIG_DEVTMPFS: is not set when it should be

Problem

emerge  sys-fs/udev

* Messages for package sys-fs/udev-200:

 *   CONFIG_DEVTMPFS:    is not set when it should be.
 * Please check to make sure these options are set correctly.
 * Failure to do so may cause unexpected problems.

Solution

 You have to check the following  option in your kernel config :

 Device Drivers-> Generic Driver 
* Maintain a devtmpfs filesystem to mount at /dev.  

 For getting the devfs automatically mounted at boot time it makes sense to also enable the option  
* Automount devtmpfs at /dev