Implement deviceinfo under /usr/share/ and drop /etc/boot/cmdline.txt

/etc/boot/cmdline.txt was simply created as a way to append to the commandline by users on the lack of a solution to properly modify deviceinfo variables. As finally agreed in [1], the way we plan to support that necessary feature is by creating a new "deviceinfo_kernel_cmdline_append" variable. This makes the need cmdline.txt fine unnecessary.

[1] https://gitlab.com/postmarketOS/pmaports/-/issues/1836#note_1293968974

The good thing of the deviceinfo relocation, is that it's backwards compatible :D

Revives !25 (closed) (I removed my kernel fork without realizing), addressing the comment of the location of the deviceinfo file.