hmm replace patch-cmdline by append-dtb (you have to do it anyway) and instead of lzma try gzip
KERNEL_INITRAMFS := kernel-bin | append-dtb | gzip | tplink-v1-header
I don't know if # is ok right there, maybe remove the | lzma
I have the "go" command but I don't know how to remove the low level initialization.... I can compile u-boot from tp-link sources, but I am not so skilled to edit the u-boot sources blindly...
About the JTAG, there are the pins... but probably is connected to the broadcom cpu and I don't have an spi programmer... I should buy it...
I am still using the old ar71xx target... when it will boot correctly I'll use the ath79 target... However I'll try your string....
[OT] Just for clarification... I have two QCA9550 board: this one and the Sitecom WLR-8100... For what concern the ath79 testing I am using the Sitecom... When I'll have some spare time I'll also test your latest commit 
[/OT]
ah sorry, I thought you're using the ath79 (you should)
You could also try it firstly with a "normal" build, but most devices will hang with a second low level init...
What will you do if your tp-link bl will boot that you can boot your image?
Don´t know what throws this lzma error.
I had good experiences with this cheap CH341A spi programmer and flashrom as software.
And probabl something like this...
With this clamp i could read and write a wr1043v4 without desoldering, but cheaper/low power devices as wr841v8 does not work without desoldering.
My hunch is still, like I wrote above, about the compression parameters used in creating the LZMA image. In addition to the dictionary size, you might try removing the three other non-standard parameters "-lc1 -lp2 -pb2" in order to see if the compressed image could work.
Success!!! 
Eventually the problem was: wrong header version!
The position for the kernel size is different in the v3 header... So previously the modem read 80 06 00 00 as kernel size... Approximately 2GB! 
Apparently this is the only ar71xx device to require the Tp-Link header v3... while other ar71xx needs the safeloader or the the v1....
Do you think I should create the patch for ar71xx or ath79 or both?
However thank you all for the help!
@juppin Fun fact: I bought the spi programmer few hours ago on Amazon with 1-day delivery!! 
It will be useful the next time!
Attention is slowly shifting from ar71xx to ath79, so if you think you can support the complete device (to the extent possible, e.g. I don't expect xDSL modem functionality to ever get supported) in reasonable time, I'd concentrate on ath79 (and kernel 4.14) only, but ar71xx hasn't been formally closed for device additions yet.
Ok, I'll try to do both in this week-end.. Next weeks I'll be practically offline due to exams...
For what concern the xDSL part, this modem (but also vr2600 and few other) could be "supported" in theory, because of the two cpus... for example I can restart the QCA9550 while the broadcom is still alive and working...
We should "just" find the way the two cpus send commands to the other. Unfortunately I don't have an adsl line anymore... so I can't test it... but I'll inform you about my findings
in my case the error message was slightly different:
Uncompressing Kernel Image ... LZMA ERROR 1 - must RESET board to recover
tftpboot was requesting default image name to load at address 0x80100000 so i tried load my built image to that address and got the message above.
however if on boot select 1. load image to SDRAM it prompts for ip addresses and image file and loads it to 0x80a00000 and from that address extraction goes fine and boot continues.
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