UBUNTU: SAUCE: async_populate_rootfs: move rootfs init earlier

Check to see if the machine has more than one active CPU, if it does
then it is worth starting the decode of the rootfs earlier.

Signed-off-by: Andy Whitcroft <apw@canonical.com>

(cherry picked from commit a7baa716

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