Makefile.inc: Use -Og when compiling with GDB support

From GCC's documentation:

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: I9a3dadbf8e894cb28e29d7b2f4e9add252e7bbb3
Signed-off-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Signed-off-by: Scott Duplichan <scott@notabs.org>
Reviewed-on: http://review.coreboot.org/8689
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
2 files changed