kmod: does not support static builds

Since kmod 14, the support for building a static library has been
removed completely from kmod. Therefore, we mark kmod as
!BR2_PREFER_STATIC_LIB, as well as all its reverse dependencies, which
includes the option to use the "udev" /dev management method.

[Peter: show comment for udev when BR2_PREFER_STATIC_LIB is enabled]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>