Clean up compiler optimization variable DEBUG_OPTIMIZATION

Clean up compiler optimization variable DEBUG_OPTIMIZATION

Remove DEBUG_OPTIMIZATION, set "-Og ${DEBUG_LEVELFLAG}" to
SELECTED_OPTIMIZATION when debug optimization is enabled (fragment
core/yocto/debug-optimize)

Use SELECTED_OPTIMIZATION to instead of DEBUG_OPTIMIZATION
for debug optimization tuning configuration

Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>