Makefile: avoid false positive -Wmaybe-uninitialized

When compiling with -Og gcc reports false positive -Wmaybe-uninitialized as
reported in https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78394

.

Silence these warnings when building with CONFIG_CC_OPTIMIZE_FOR_DEBUG.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>