Add support for unlocking multi-device root

[graywolf]

Add cryptroo[0-9]= family of options to allow unlocking multi-device
root filesystems, which is needed for btrfs raid1.

[ncopa]

this seems to solve same problem as #54 tries to solve, but in different way?

[graywolf]

Yes, I think the problem solved is similar (same?). But when I decided to try alpine, #54 was not upstreamed (still is not) and since I did not like few design decisions (not being able to specify different options for different cryptoroots, handling anything ending with *.img as LUKS-encrypted file), I've put together different pull request doing this in backwards compatible way.

EDIT: Thinking about it, my version can also do what #54 is doing, except for the autoclose of the key device. That can either be added (cryptautoclose=true?) or just handled in openrc service added to boot level.

[graywolf]

I've rebased on current master.

[graywolf]

Hello,
is there anything I can do to help push this forward?

W.