This uses a "deviceinfo_create_initfs_extra" to allow including
initramfs-extra files in the initramfs and skip creating a separate
initramfs-extra archive when it's set to "false".
If this variable is unset, mkinitfs uses a default value of "false".
This library has a convenient "source file" method designed for sourcing
shell envs and returning values set in them. deviceinfo's syntax every
where else seems to be "whatever sh can 'source'", so using this library
seems a lot nicer than trying to implement a parser/interpreter here
(and almost certainly missing corner cases, functionality, etc)
[ci:skip-build]: already built successfully in CI
