bootstd: Allow enabling BOOTSTD_FULL without needing EXPO

It is sometimes useful to have one without the other, e.g. on a device
without a display, since at present the expo feature requires CONFIG_VIDEO
to be enabled.

Update the Makefile and bootflow command to support this, as well as the
EXPO dependency.

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