How to set up root in custom firmware if there is no sudoers?

Question

linux

I compiled the firmware for DIR-300C1 from the sources, before the build I can edit any configuration, how can I provide root for the admin user, if sudo is not there at all? I wrote about this before, but without an answer, please help.
Links:
Sources
Examples of configurations in etc.zip etc.
I only took up this because it became interesting.

Answer 1

Put him in the wheel group, after which he can execute su -