Now that I have my serial console working, I've been able to install a lede build downloaded from the lede downloads tree: lede-17.01.0-rc2-r3131-42f3c1f-ipq806x-R7800-squashfs-factory.img
However, having installed this image, when I try to install a sysupgrade that I've built from a tree updated about an hour ago, I get this (in the Luci upgrade UI):
The uploaded image file does not contain a supported format. Make sure that you choose the generic image format for your platform.
If I try to flash the sysupgrade that's equivalent to what's already running, it accepts it as a valid image (I didn't actually proceed to flash it). Am I missing something? Should I not expect a build from the head of the tree to be usable? There's such a long stream of discussion on this thread that although I am sure the answer to my question is in there somewhere, I haven't been able to find it.
I'm wondering if some of this stuff might belong on the wiki...
(BTW, I should say that the factory image from the same build also fails; what I get in this case is:
MODEL ID on image: D7800
Firmware Image MODEL ID do not match open source firmware ID
131072 bytes read: OK
HW ID on board: 29764958+0+128+512+4x4+4x4+cascade
HW ID on image: 29764958+0+128+512+4x4+4x4
Firmware Image HW ID do not match Board HW ID
Board HW ID mismatch,it is forbidden to be written to flash!!)