Drop background readahead on boot
_Originally created by @intrigeri on [#15915 (Redmine)](https://public-redmine-archive.tails.boum.org/code/issues/15915)_
Starting Tails 3.9 from DVD is twice slower than 3.8 (3:55 vs. 2:12 on
my hardware), presumably because my “common sense” (sic) did not make me
filter out enough stuff from the SquashFS sort file. I did not measure
things nor look at the bootchart but it seems as if the X.Org startup
was blocked for 2 minutes, waiting for the background readahead to
complete, which is kinda ironic.
I think we should:
1. keep the SquashFS sort file for now, hoping it is still useful
2. try removing the part about `BG_FILES` from
`config/chroot_local-includes/lib/live/config/0000-readahead` and
measure boot time from DVD and USB, both in a problematic case (3.9)
and optimal case (like the squashfs.sort used for 3.8)
3. if removing background readahead helps, try removing the foreground
readahead as well and measure things again
Feature Branch: feature/15915-remove-readahead
### Attachments
* [dmesg.txt](https://redmine.tails.boum.org/code/attachments/download/2237/dmesg.txt)
* [tails-amd64-feature_15915-remove-readahead-3.12-20190106T1143Z-79907a506c.buildlog](https://redmine.tails.boum.org/code/attachments/download/2238/tails-amd64-feature_15915-remove-readahead-3.12-20190106T1143Z-79907a506c.buildlog)
### Related issues
- [x] **Blocked by** tails/tails#16134
issue