linux: don't automatically set uevent_helper with mdev /dev management

When mdev /dev management is chosen in the buildroot configuration, the
Linux configuration is updated automatically to set option
CONFIG_UEVENT_HELPER_PATH to "/sbin/mdev". However, the help text of this
option explicitly recommends not setting this option due to large
performance impact during boot (experienced first hand by the reporter ánd
author).

The mdev startup script S10mdev already sets the helper during userspace
boot, which will make sure mdev is working correctly.

Fixes bug #6596: https://bugs.busybox.net/show_bug.cgi?id=6596



Reported-by: Andreas Koop <andreas.koop@zf.com>
Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>