Ok, probably yesterday I did something stupid: I created /home/user/.profile to start keychain, and it worked as desired.
Now the device won't boot: I see the nokia logo, the progress bar arrives at the end and it is stuck there.
I suspect something in the init scripts is spawning a user shell, which is executing my .profile, which in turn is waiting for the introduction of my ssh passphrase.
Unfortunately neither usb nor the network is active on the tablet (I don't see it associated in the access point an it isn't asking for a lease) so I cannot ssh to it to amend the mess.
Is there a way out of it without reflashing?
TIA

Ok, I could finally access it through ssh (I used "flasher --boot single" but I doubt that it really did anything) and, as I thought, it was stuck waiting for the passphrase. I removed .profile and now it boots as it should.

For the record: now I check in .profile than TERM is xterm before running keychain, while running the init scripts TERM is set to linux so it won't get stuck asking for the passphrase.