basic_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: I71892fd950f422d77cb28c2a9fb5391ca151ff34
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>