UBUNTU: SAUCE: Make populate_rootfs asynchronous

The expansion of the initramfs is completely independant of other
boot activities.  The original data is already present at boot and the
filesystem is not required until we are ready to start init.  It is
therefore reasonable to populate the rootfs asynchronously.  Move this
processing to an async call.

This reduces kernel initialisation time (the time from bootloader to
starting userspace) by several 10ths of a second on a selection of test
hardware particularly SMP systems, although UP system also benefit.

Signed-off-by: Surbhi Palande <surbhi.palande@canonical.com>
Signed-off-by: Andy Whitcroft <apw@canonical.com>

(cherry picked from commit 29f1e2fd

 for backporting)
Signed-off-by: Bryan Wu <bryan.wu@canonical.com>
Signed-off-by: Andy Whitcroft <apw@canonical.com>