This changes boot_config_load_and_select() to also take the root path as
input, just like the ESP and XBOOTLDR path.
This has the benefit of making the whole file independent of bootctl.c,
which means we can link it into a separate test, and is preparatory work
for a follow-up commit.
These operations to quite different things, they just share 2 common
funcs. Let's split them out into separate files.
This also splits up verb_list() into separate calls for the three
operations. This actually fixes issues, as for status/list we want
"unpriv" ESP discovery logic, but for the other two we really should
have privileged discovery logic.
This is preparation for adding "bootctl link" later, but this makes
sense either way, I am sure.
Lennart Poettering