[FS#1086] can't upgrade from unifi ap stock BZ.v3.2.12 to lede via syswrapper.sh update2

[LEDE Bugs]

A new Flyspray task has been opened.  Details are below. 

User who did this - Moritz Fago (Moritz) 

Attached to Project - LEDE Project
Summary - can't upgrade from unifi ap stock BZ.v3.2.12 to lede via syswrapper.sh update2
Task Type - Bug Report
Category - Base system
Status - Unconfirmed
Assigned To - 
Operating System - All
Severity - Medium
Priority - Very Low
Reported Version - lede-17.01
Due in Version - Undecided
Due Date - Undecided
Details - Supply the following if possible:
 - Device problem occurs on
Ubnt unifi ap
 - Software versions of LEDE release, packages, etc.
stock 3.2.12
 - Steps to reproduce
   1. upload lede-17.01.4-ar71xx-generic-ubnt-unifi-outdoor-squashfs-factory.bin to device
   2. try syswrapper.sh update2
   3. fail
   4. do fwupdate.real -c /tmp/fwupdate.bin (to check), complains about "invalid file: BZ.ar934x.v6.0.0-OpenWrt-r3560-79f57e422d (not quoted)
   5. openwrt-15.05.1-ar71xx-generic-ubnt-unifi-squashfs-factory.bin works: BZ.ar7240.v6.0.0-OpenWrt-r48532
 The String beginning with BZ seems to be the magic number at the beginning of the file. For comparison: UBNTBZ.ar7240.v3.2.12.2920.150531.1645 from a stock fw

More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=1086