deviceinfo: replace implementation with mvdan/sh (MR 52)

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

pkgs/deviceinfo/test_resources/deviceinfo-first

@@ -1,2 +1,3 @@
+deviceinfo_format_version="0"
 deviceinfo_initfs_compression="gz -9"
 deviceinfo_mesa_driver="panfrost"