linux: change Device Tree prompt

The current prompt seems to imply that we want to add Device Tree
support to the Linux kernel:

    [*] Device tree support

But what it really means is that Buildroot will build a DTB.

Change the prompt so that it is obvious that this is the intended
behaviour, and users do not get mislead as to why Device Tree support is
not automatically added to their Linux kernel.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>