build: use -Og instead of -O0 for CONFIG_DEBUG

From the GCC manual for -Og option, introduced in GCC
4.8:

Optimize debugging experience. -Og enables optimizations
that do not interfere with debugging. It 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

Change-Id: I65751cf3117bc2e6f70a7008b170126160cfa48c
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>