base-files: add sysupgrade -k to save list of pkgs

base-files: add sysupgrade -k to save list of pkgs

When '-k' is used, sysupgrade inserts into backup a new file
/etc/backup/installed_packages.txt which contains pkgname and origin (rom,
overlay, unknown) without touching rootfs.

It's mainly used to reinstall all extra packages:

 # opkg update
 # grep "\toverlay" /etc/backup/installed_packages.txt | cut -f1 | xargs -r opkg install
 # rm /etc/backup/installed_packages.txt

Signed-off-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>
(backported from 5cb1dce542a6a13fd250e9ef9c8290568c610c39)