After some time i had some trial and error....lots of reading i've succeeded in uploading another firmware version to the Stretch 2.0 & Smile P1 using a TFTP server and the U-Boot bootloader.
First you need Plugwise firmware, read this:
"How to Obtain PlugWise firmware + crawler scripts! @ http://domoticx.com/plugwise-firmware-downloaden
Well...i have downloaded already all possible firmware versions, so no need to do it yourself!
Things you need to upload your own firmware image:
1) Hardware: UART > USB stick + some wires
2) Software: PuTTY tools
3) Software: TFTP server
Connect UART @ http://domoticx.com/plugwise-smile-p1-v ... -uart-ttl/
Upload firmware trough the bootloader @ http://domoticx.com/plugwise-smile-p1-f ... ootloader/
Good to know: Smile has failsave mode till firmware v1.1.9
Connect UART @ http://domoticx.com/plugwise-stretch-2- ... -uart-ttl/
Upload firmware trough the bootloader @ http://domoticx.com/plugwise-stretch-2- ... ootloader/
Good to know: Stretch has failsave mode till firmware v1.0.38
Please note you may lose warranty when you do the above!
So, the next level is to compile our own firmware for the Smile/Stretch with OpenWRT