Code review comment for ~waveform/ubuntu-release-upgrader:remove-uboot

I found the function _remove_uboot_on_rpi a bit hard to read and had to scroll up and down multiple times because the file is read, then some other functions are defined, and then the file is passed to those functions. It might be easier for others to read if that was restructured a bit.

I can't really comment on the correctness of the boot config contents but as a release upgrader quirk this seems good to me.

I've some additional comments in-line.