base-files: sysupgrade: always setup overlay when creating backup

base-files: sysupgrade: always setup overlay when creating backup

Setting overlay while creating backup allows including extra files in
archive without actually writing them to flash. Right now this feature
is limited to /etc/backup/ directory and is used only for including
installed_packages.txt.

Extend this solution to make it more generic:
1. Always mount overlay while creating backup
2. Overlay whole / to don't limit it to /etc/backup/

This allows including any additional files in backups and adding more
sysupgrade features.

Cc: Luiz Angelo Daros de Luca <luizluca@gmail.com>
Cc: Christian Marangi <ansuelsmth@gmail.com>
Cc: Jo-Philipp Wich <jo@mein.io>
Cc: Jonas Gorski <jonas.gorski@gmail.com>
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>