Dear Nuno Cardoso,
In message x2j3cf2debb1005060055w943eecd4t9a87659b0badf2e@mail.gmail.com you wrote:
When I change the root password I'm using the command that you specify "passwd root" and enter a new password. After that, I execute the "exec init" command in U-Boot shell to boot all the linux system. At login, I put the new password, but I cannot log!!!!!
After a restart, I stop the u-boot process to start shell (init=/bin/sh) and cat the /etc/shadow file, and the root password doesn't change (is the old). What I'm doing wrong?
Probably you are missing a step to write the changes to the password data back to some persistent storage; when re-booting, you are re-loading the old, unmodified data.
We don't know your system and it's file system layout. We don't know if it provides any form of persistent, changable storage for these data, and where.
When booting with init=/bin/sh the whole system is at your hands - explore it and find out.
Eventually the data is in a read-only file system and cannot be changed at all - then you can try to upload the file system image, undpack it, modify it, repack it and install it on your target. Of course there is a chance that you brick the system that way, so you better know exactly what you are doing.
We do not know your systeme. We CANNOT help you. And all this is off topic here.
Best regards,
Wolfgang Denk