Code review comment for lp://qastaging/~pundiramit/linaro-android-build-tools/access_private_manifests

Ok. With this new patchset, I changed how I was handling SOURCE_OVERLAY variable. Since SOURCE_OVERLAY variable is something which may or may not be directly used by jenkins for build purpose, I'm not playing around it anymore.

What I did this time is that I introduced a new flag SOURCE_OVERLAY_OPTIONAL which is set to "1" to make overlay optional. This SOURCE_OVERLAY_OPTIONAL flag will be toggled to "0" when we explicitly set overlay using "-o <overlay>" switch. To protect S_OVERLAY portion of the script, I have put relevant conditional checks for S_O_OPTIONAL flag.

Let me know if you have any concerns on this one.