apk: provide csum for static conffiles

apk: provide csum for static conffiles

For non-overlay configuration we need checksum for config file that
weren't modified by the user. For OPKG in sysupgrade we check the status
file for the Conffiles: entry of every package. this entry contains
checksum for every static file that the package contains.

Provide the same info for APK by creating a conffiles_static file and
parse this file on sysupgrade for non-overlay configurations.

This is also used by the sysupgrade -u option to exclude non-changed
files from the final backup.

Link: https://github.com/openwrt/openwrt/pull/15543
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>