rootfs_ipk.bbclass: move rootfs postprocess command

rootfs_ipk.bbclass: move rootfs postprocess command

I ran into a problem yesterday where the ROOTFS_POSTPROCESS_COMMAND
started failing after I turned off ONLINE_PACKAGE_MANAGEMENT. It seems
the problem is that if package management is turned off, then the opkg
directory gets deleted. Subsequent opkg commands in the
ROOTFS_POSTPROCESS_COMMAND fail to open the lock file because the
directory is gone.

This patch simply moves the postprocess command above the destruction of
the opkg directory to allow any such commands to complete successfully.

Acked-by: Marcin Juszkiewicz <marcin@juszkiewicz.com.pl>