image.bbclass: run postinst scripts once, at rootfs time

image.bbclass: run postinst scripts once, at rootfs time

This patch will allow the repeating postinst scriptlets to be run
only once, on host, at do_rootfs time. This will lower the time for
rootfs generation and, also, instead of running some time consuming
scriptlets at target's first boot, we will do on the host.

Signed-off-by: Laurentiu Palcu <laurentiu.palcu@intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>