feat: add .driverpostsync hook support

[amolani]

Summary

Adds support for an optional .driverpostsync script, enabling patchless driver injection based on hardware profile matching without modifying the existing postsync workflow.

Changes:

• linbofs/usr/bin/linbo_sync — source <imagebase>.driverpostsync after the regular .postsync (same context: mounted OS at /mnt)
• linbofs/usr/bin/linbo_download_image — add driverpostsync to the download loop so the file is fetched from the server into /cache/

How it works

1. A driver management tool generates .driverpostsync scripts per image in /srv/linbo/images/<image>/
2. linbo_download_image downloads the script to /cache/ alongside .postsync and .reg
3. linbo_sync sources it after the regular postsync — drivers get copied into the synced OS

Testing

Tested on a live linuxmuster 7.2 setup:

• Clients sync normally when no .driverpostsync exists (no-op, [ -s ... ] guard)
• When .driverpostsync is present, drivers are correctly injected
• No impact on existing .postsync behavior