Fix booting from mdadm raid.

Hi.

As subject states, grml can't boot from (local) mdadm raid.

The reason is simple: because hostname in initramfs (it's empty) does not match with the hostname of installed system, mdadm assembles arrays with different names, and there's no md device, which 42_grub has added to live-media=. E.g. when installed system has /dev/md/0 and hostname "debian11", mdadm from initramfs creates /dev/md126 and symlink /dev/md/debian11:0, but neither of them matches with /dev/md0 added into live-media= by 42_grub.

I've tried (too) many workarounds, but none of them worked:

adding live-media= to CUSTOM_BOOTOPTIONS does not work, because 42_grub creates another one and effectively overwrites it.
ip= and (grml's) hostname= kernel command-line option works too late, after mountroot stage at which initrd tries to assemble raid, so can't help.
Neither works md= kernel option, which seems used for older mdadm metadata formats and now ignored completely.
Nor (surprisingly) adding fixed mdadm.conf (with HOMEHOST <ignore> or HOMEHOST real_hostname) to initramfs. This one because with boot=live initramfs deletes mdadm.conf (in check_dev() function from lib/live/boot/9990-misc-helpers.sh) before calling to scripts/local-block/mdadm (well, the reason is explained there and seems reasonable).

So, patching 42_grub seems the only way.

There're probably two ways of fixing this: allowing to overwrite live-media= in CUSTOM_BOOTOPTIONS or just fixing autodetected mdadm array name. Here is implementation of the second one.

grml / grml-rescueboot

Fix booting from mdadm raid. #11