Support multipath root / boot device

[agelwarg]

We have built a software appliance based upon k3os, and have use-cases where the installation requires boot-from-SAN (i.e., multipath support). These changes are the basis for how we have met these requirements. For installation, we boot into the live installer without multipath enabled and install to any one of the available paths. Upon reboot into the installed system, we enable multipath while in MODE=disk before pivot_root and calling init with MODE=local. The end result is such that our root fs is mounted on the multipath device / partition as seen below:

Filesystem                    Size  Used Avail Use% Mounted on
/dev/mapper/mpatha-part1      4.6G  1.1G  3.3G  25% /

Since not everyone would want this built in, maybe we could/should make these changes optional based on some build-time paramater(s).

[dweomer]

I haven't dealt with multi-path directly (other than including the libs way back when to make openebs and longhorn work correctly iirc). This seems rather ... delicate.

[agelwarg]

I think we probably can (and should) add more testing around these particular steps.

With that said though, I don't think that everyone will want extra packages and/or modules continuing to bloat the size -- I think I've seen a similar comment here before about not wanting to keep adding more and more packages when most/many people don't want them. The value here is tracking how to make it work, in one place, sharing the info for those that do need it. However maybe this is a build-time decision via a flag/switch/etc? Then somewhere in the docs we could list such flags and explain what they impact. Additionally, for common ones maybe you could provide alternative builds directly for download.