thanks @giuliomoro and Liam
ok, ran the 'new' script on the Salt, and all worked, flashed to emmc, all fine
as a test (on a full fat bela, where i could get to usr ok) ) , I did the dd trick, and that also worked fine.
(perhaps thats easier, and potentially less 'dangerous' , as better to have the emmc unbootable, rather than potentially damaged? I'll let you muse over this )
separately i had 2 minor issues with 'make_salt.sh'
a) it uses hostname to determine its on a bela , and Id renamed my board to salt.
(to differentiate from my belas, which I call bela1, 2, 3 and belamini)
b) I got "An unexpected error happened while making your board salty"
(Interestingly ~/.bela/belaconfig , said BOARD = Salt)
the cause was from a previous build on board_detect.
so was fixed with
Im not sure how exactly how this happened... but, its somewhere in this process ...
so because i had a number of belas to update, what i did was:
a) create a sdcard image, boot it up, called update_board , setup up usb networking, tested it on 'bela1'
b) flashed the image to bela1, changed hostname to bela
c) removed sdcard, and did the step (b) for other boards, including salt
(actually for salt, when it failed, I also ran update_board again, which didnt fix it)
not quite sure what the error was...
on the sdcard (that i used to flash emmc on salt from) , board_detect.cpp was the latest, and it had obviously been built. there was no ~/.bela/belaconfig
but given forcing a rebuild of board_detect 'cured' the issue, Im guessing the issue was...
board_detect had been previously built, with an old board_detect.cpp , but the binary generated was now fairly 'new' (i.e. june 2018) , so when I did update_board, i got the new board_detect.cpp but it was still older than the binary build, so make decided it didn't need rebuilding.
so I guess what needs to happen is update_board should be doing a 'make clean' which removes all the resources/tools (Ive not checked, perhaps its just missing this one target!)
anyway , all cool now, everything is up to date
thanks for your help