build: propagate override information to .packageinfo

Packages overridden with "./scripts/feeds -f" will show up with an
"Override: <original-package-name>" in .packageinfo.

This will be useful to create a corresponding option in .config and
use this information during the build process.

Signed-off-by: Mathieu Olivari <mathieu@qca.qualcomm.com>

SVN-Revision: 44335