board_f: Use IS_ENABLED instead of #ifdef in initf_bootstage()

The current implementation makes it look like the 'if (from_spl)' part is
dead code because these features are not enabled for sandbox. We could
enable it for sandbox_spl, but this is not done yet (it requires sharing
memory between SPL and U-Boot proper which is in fact supported).

It is probably nicer to avoid #ifdef anyway. Change it.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reported-by: Coverity (CID: 163244)
Fixes: 824bb1b4 (bootstage: Support SPL)