openwrt: add patch fixing sysupgrade on Xen DomU

Patch tested on v2016.1.x branch with Freifunk Magdeburg firmware on
Debian Jessie amd64 Xen host. See the same patch in our gluon fork here:
https://github.com/FreifunkMD/gluon/blob/fix-sysupgrade-xen/patches/openwrt/0058-x86-fix-platform_export_bootpart-for-Xen-virtual-disks.patch



Sysupgrade was tested successfully by manually making the change before
upgrading in the filesystem of the running node and upgrading to the
fixed FFMD experimental build in config mode via expert settings in
webgui.

Patch also submitted to OpenWRT trunk already.

Signed-off-by: Alexander Dahl <alex@netz39.de>