After switching from Ubuntu 12.04 to 14.04 I've noticed my self compiled kernels would not boot well. There was always an error that a device (root / and swap) could not be mounted. Also I could no longer mount removable devices, such as USB sticks.
It turns out this is because Ubuntu now requires in kernel automounter support version 4. The config entry is CONFIG_AUTOFS4_FS=m. Then everything worked again.