bootm: Require boot function only if it is about to be used

The original bootm code (before commit 35fc84fa) did not check for a valid
boot function in the subcommand case, which was incorrect.

This check was introduced in all cases, but in fact we should only check
for the function when we need it. Otherwise in some cases the check fires
before the OS type is known.

Signed-off-by: Simon Glass <sjg@chromium.org>