initramfs-tools: Please support mounting of /usr in the initramfs

Imported from Debian bug http://bugs.debian.org/652459:

Package: initramfs-tools
Version: 0.99
Severity: normal
Tags: patch

Hi,

In order to make the libraries and binaries in /usr available during
early boot, it would be desirable to be able to mount /usr in addition
to the rootfs inside the initramfs.

This patch implements a generic solution into two parts
1) Generation of /etc/fstab in the initramfs, including the rootfs
   and all the filesystems desired to be mounted
2) In local mountroot(), rather than just mounting the rootfs, loop
   over all mountpoints in /etc/fstab and mount them.

Note that this can replace the ROOT= option with the fstab entry alone,
though if set, ROOT= supercedes the root entry. If the host /etc/fstab
does not contain a root entry, a dummy one is generated to ensure that
the rootfs is always the first entry in the initramfs fstab. This
also means that the rootfs may be mounted with the options in
/etc/fstab.

This isn't intended to apply directly--it's more of a proof of concept.
I've tested the fstab generator; I haven't yet tested local mountroot().
Not being totally familiar with the initramfs internals, it might also
need:

- inclusion of modules for the additional filesystems
- ability to NFS mount filesystems other than the rootfs, i.e.
  it might need nfsmount for local (not that this is a useful use
  case)

Also note that the initramfs mount option (patch sent upstream) is not
yet in mount(8), and it would need to be before the option could be
used.

Regards,
Roger