board: sandburst: delete FORCEBUILD

We had switched to Kbuild, so we do not need to
delete sandburst board files at every build.

U-Boot conventional build system did not check the
update of command line option, -DBUILDUSER.

Kbuild can handle it nicely and re-builds object files
when command line options are changed.
(The file ".*.cmd" stores the information how the file
was generated at the previous build.)

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>