busybox: disable MTD utils in default configuration

The MTD utils require MTD headers in the toolchain, and in several
external toolchains, they are not present. In order to make the build
work by default, let's disable the MTD Busybox applets in our default
configuration.

Fixes bug #1669

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>