build: Use stable file ordering in sysupgrade.tar

build: Use stable file ordering in sysupgrade.tar

The file ordering in the current sysupgrade depends on the order of file in
a filesystem. This is often already in a sane (alphabetical order) but this
is not always the case. For example, the OpenWrt build servers return a
different ordering.

This breaks the reproducibility of the sysupgrade tarballs significantly
and also resulted in images which cannot be used for upgrades on devices
like the OpenMesh A42/A62.

Signed-off-by: Sven Eckelmann <sven@narfation.org>