git: mkinitrd - add tool and infrastructure for initrd

[Alex Hornung]

: What can these ramdisks be used for?

Fair question. For nothing really yet, but the intention is to be able
to mount root from lvm2 volumes, crypto volumes and all other sort of
device mapper volumes. They could also be used to enable iscsi root
support and any other mountable file system.

What happens is that the early userland init (oinit) spawns /etc/rc
(which is taken from /usr/share/initrd/etc on calling mkinitrd), which
then needs to mount the future root file system on /new_root and null
mount the /dev mount onto /new_root/dev. The current /etc/rc does this
by assuming I want to mount my default hammer system on da0s1d, but
the final intention is to add a much better early rc system that will
scan for lvm volumes amongst others and then read some parameter of
what disk should finally be mounted.

Hope that helps,
Alex