rootfs-postcommands.bbclass: make opkg status reproducible

rootfs-postcommands.bbclass: make opkg status reproducible

opkg stores the current time as Installed-Time in its status file when
installing packages to the rootfs. Make this reproducible by replacing
Installed-Time with ${REPRODUCIBLE_TIMESTAMP_ROOTFS}, which then also
matches the files' datestamps.

Based on OpenWrt's approach for the issue [1].

[1] https://github.com/openwrt/openwrt/blob/main/include/rootfs.mk#L103

Signed-off-by: Jonas Gorski <jonas.gorski@bisdn.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 61a9b1b1cb618ce90ba7886036f41263075c07df)
Signed-off-by: Steve Sakoman <steve@sakoman.com>