qemu_cortex_m3: 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: I54e94ff2cc2d2bfbfb8a016ae49b4b4d13f25dd2
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>