board: altera_max10: 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: I0fb1105f19149b0e17c45455368ddf0ef75e5165
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>