qemu_x86: 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: I742bc695e8a1efb34003e651f57c8aba4fb798f6
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>