overlayroot should support multiple lowerdir mounts

[Ryan Harper]

As of 3.19+, overlayfs supports multiple lowerdir mount points to enable merging multiple read-only layers together which is useful for composing multiple layers together and then imposing an specific upper layer (say tmpfs).

From the kernel documentation[1]

| Multiple lower layers
| ---------------------
|
| Multiple lower layers can now be given using the the colon (":") as a
| separator character between the directory names. For example:
|
| mount -t overlay overlay -olowerdir=/lower1:/lower2:/lower3 /merged
|
| As the example shows, "upperdir=" and "workdir=" may be omitted. In
| that case the overlay will be read-only.
|
| The specified lower directories will be stacked beginning from the
| rightmost one and going left. In the above example lower1 will be the
| top, lower2 the middle and lower3 the bottom layer.

To enable this in overlayroot, we'd extend the the overlayroot parameter to take multiple lower= parameters:

Take for example a current command line used to boot a cloudimg and use a tmpfs backed rw layer:

root=LABEL=cloudimg-rootfs overlayroot=tmpfs

If we had a second device/partition that we wanted to stack on top of the root fs, we'd pass this on the command line

root=LABEL=cloudimg-rootfs overlayroot=tmpfs,lower=LABEL=myoverlay

which would:

1. ensure that we mounted LABEL=myoverlay on the root filesystem (say /media/lower1-ro)
2. then run the following overlay mount

mount -t overlay -o lowerdir=/media/lower1:/media/root-ro,upperdir=/media/root-rw/overlay,workdir=/media/root-rw/overlay-work

Giving us a final stack of:

[tmpfs]
[LABEL=myoverlay]
[LABEL=cloudimg-rootfs

We also may want to stack more than one extra lower, specifying top-most layer first, on to the lower layers.

root=LABEL=cloudimg-rootfs overlayroot=tmpfs,lower=LABEL=myoverlay,lower=iscsi:192.168.50.1::::iqn.2009-06.dracut:target0,lower=/dev/vdc

resulting in:

[tmpfs]
[LABEL=myoverlay]
[iscsi]
[vdc]
[LABEL=cloudimg-rootfs]

1. https://www.kernel.org/doc/Documentation/filesystems/overlayfs.txt