opkg: support for upgrading packages

This patchset adds support for the 'opkg upgrade' command.  This will
allow someone who needs to update any package already installed to the
latest, if available.

On the version of OpenWRT used for testing, the 'opkg install' command
will also upgrade a package and 'opkg upgrade' will also install new
packages.  Rather than removing the check to see if a package is
installed before running 'opkg install' again, I chose to write this new
option to not rely on that behavior in the event that opkg changes in
the near future.