basic_minuteia: Remove empty board file

Use a kbuild trick to force built-in.o to get built even if there isn't
any code so we can link properly.  This is cleaner than keeping around
files that don't do anything.

We keep around board.h for now since drivers and other code might expect
it to exist.

Change-Id: I1ed09f05ea2c2d08d11fc742d76f32c1f8f2fbbd
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>