bootstd: Correct default boot command

bootstd: Correct default boot command

The patch to relax flag requirements was not accepted[1], so we still have
to have separate bootcommands depending on CMD_BOOTFLOW_FULL.

The previous attempt at this did not work, since it used the wrong name
for the options.

Fix this and change the message to mention BOOTSTD_FULL since this affects
not just the flags, but all functionality, so is more likely what the user
wants.

Drop the useless condition on CMD_BOOTFLOW_FULL while we are here.

[1] https://patchwork.ozlabs.org/project/uboot/patch/20230329071655.1959513-2-sjg@chromium.org/

Signed-off-by: Simon Glass <sjg@chromium.org>
Fixes: a91492b6e9c ("bootstd: Provide a default command")