Run "./post-build.sh <target root> <build info>" in post-build stage to:
1/ apply extra partitions to /etc/fstab.
2/ add build-info to /etc/os-release.
Change-Id: Icdc7049bce389c38d91d348cda803669be9d1cb2
Signed-off-by: Jeffy Chen <jeffy.chen@rock-chips.com>