Add option to use -Og

This adds support for using -Og when building U-Boot. According to the
gcc man page:

> -Og should be the optimization level of choice for the standard
> edit-compile-debug cycle, offering a reasonable level of optimization
> while maintaining fast compilation and a good debugging experience.

This optimization level is roughly -O1 minus a few additional
optimizations. It provides a noticably better debugging experience, with
many fewer variables <optimized out>.

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Reviewed-by: Simon Glass <sjg@chromium.org>