This patch got lost in the OpenWrt -> LEDE transition and fixes flash
writes on new UBNT PicoStations.

Backport another sysupgrade patch.

The staged sysupgrade allows to properly unmount the rootfs before writing
the new partitions. This will fix upgrades losing configuration when
parition sizes change on x86 and similar image types.

Fixes meshing over 802.11s.

This will ensure that gluon-site and gluon-autoupdater are rebuilt
correctly when GLUON_RELEASE or GLUON_BRANCH change.

The ar71xx-tiny target sets CONFIG_CLEAN_IPKG, which would delete opkg
control files required for user creation. Fix image generation and the user
creation script.

Fixes #1012