buildroot

Makefile: don't run "menuconfig" automatically

Since forever, we run 'menuconfig' automatically on an unconfigured
tree. However, this does not help users that much:
- If they read the documentation, they should already know to run
  make menuconfig first.
- If they haven't read the documentation, dropping them in menuconfig
  isn't very helpful.
- It's a likely that the user didn't intend to be in an unconfigured
  tree (e.g. wrong O= specified), so starting menuconfig (and polluting
  this wrong O= directory) is not very helpful.
- It's possible that the user really doesn't want menuconfig, but
  instead needs xconfig, or some defconfig, or ...

So, instead of trying to guess what the user needs, print an error and
let the user decide what to do next.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>