Now that the most important options are exposed via menuconfig in the SDK we
can stop shipping a (possibly outdated) copy of .config from the emitting
buildroot.

Leaving out .config has two positive side effects:

 - A simple "make" run without any prior config will launch menuconfig, forcing
   interactive users to make appropriate choices

 - Buildbot systems (e.g. our phase2 builder) can issue a "make defconfig" to
   obtain a .config enabling all packages by default

Signed-off-by: Jo-Philipp Wich <jo@mein.io>