package_manager.py: support ipk incremental image generation
Add the following three functions to OpkgPM class:
- The 'dummy_install' is used to dummy install pkgs, and returns the log
of output;
- The 'backup_packaging_data' is used to back up the current opkg
database;
- The 'recover_packaging_data' is used to recover the opkg database
which backed up by the previous image creation;
Tweak 'remove' function in OpkgPM class, which the options for remove
with dependencies was incorrect.
Tweak 'handle_bad_recommendations' function in OpkgPM class:
- Fix none value check;
- Add the existance check of opkg status file;
