bootflow: Rework do_bootflow_menu() slightly

bootflow: Rework do_bootflow_menu() slightly

When building this with clang, we get a warning such as:
cmd/bootflow.c:412:27: warning: variable 'bflow' is uninitialized when used here [-Wuninitialized]
        printf("Selected: %s\n", bflow->os_name ? bflow->os_name : bflow->name);
                                 ^~~~~

And a suggestion to just initialize bflow to NULL. This would however
would be ensuring a bad dereference. Instead, looking at the function we
rework things so that when CONFIG_EXPO is not enabled (and so, no UI) we
error early and would never reach this point in the code.  Simplify the
rest slightly as well while at this.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>