bootctl: make bootspec-util.c independent of bootctl.c

bootctl: make bootspec-util.c independent of bootctl.c

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.