It lets us move the variables in cmdline.txt to
deviceinfo_kernel_cmdline. We can thereafter drop the cmdline.txt file
entirely.
By setting deviceinfo_generate_cmdline_txt we tell boot-deploy to
generate /boot/cmdline.txt from deviceinfo_kernel_cmdline. boot-deploy
can then also add information about partition UUIDs to the cmdline,
which helps postmarketos-initramfs find the correct partitions.
Fixes: https://gitlab.postmarketos.org/postmarketOS/pmaports/-/issues/3716
Part-of: https://gitlab.postmarketos.org/postmarketos/pmaports/-/merge_requests/6630
[ci:skip-build]: already built successfully in CI