bootstd: Make BOOTSTD_FULL depend on BOOTSTD

Move BOOTSTD_FULL down in the file so that it can be enabled only when
BOOTSTD is enabled. This prevents a build error if BOOTSTD is disabled
but BOOTSTD_FULL is enabled.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>