rootfs_ipk: allow do_populate_sdk in parallel to do_rootfs

Switch do_populate_sdk for the ipk package manager to use a separate target
opkg config file and separate the lockfiles restricting do_rootfs and
do_populate_sdk from running in parallel.

This way if an image recipe includes a dependency to do_populate_sdk by
default then it will run in parallel to do_rootfs saving time compared to the
sequential execution.

Signed-off-by: Michael Ho <Michael.Ho@bmw.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>