rootfs-postcommands: Try and improve ordering constraints

rootfs-postcommands: Try and improve ordering constraints

The current code is in race to see who can set things last. This isn't scalable or
sustainable and problemtic in the face of inherit ordering changes.

Move the ordering issue into the actual code execution, which isn't ideal but
the best of several bad options and at least lets us drop the anonymous python.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>