build: Allow product to override debug defconfig

Falcon GPE has a requirement that userdebug builds must have high
performance.  Allow GPE product to override the use of debug
options inherited from the common kernel.

Change-Id: I3b7ed452c5424719da65640ced5656073599a918
Reviewed-on: http://gerrit.pcs.mot.com/590151
Tested-by: Jira Key <jirakey@motorola.com>
Reviewed-by: Dmitry Sokolov <dima@motorola.com>
SLTApproved: Dmitry Sokolov <dima@motorola.com>
Submit-Approved: Dmitry Sokolov <dima@motorola.com>

Conflicts:

	defconfig.mk