OcBootManagementLib: Convert naming from External System Boot to Unmanaged Boot

[mikebeaton]

External is already used for:

• External picker (as opposed to builtin)
• External file system (as in, which icon to show in Canopy, or whether to mark entry “ (external)” in builtin picker)
• External Tool (user defined tool)
• External OS (user defined entry; it is also used for entries from boot entry protocol*)
• External Boot System

This PR suggests changing the last one to Unmanaged Boot This is a PR split off from #549. I found it easier to work on that after these changes, which is why it was included in there in the first place.

EDIT: See additional explanation in comment below.

(*I only spotted this starred point while separating out this PR. I wonder if these should really have been Other OS instead of External OS all along? Like non-macOS, non-Windows blessed entries?)

[mikebeaton]

Note that OC_BOOT_EXTERNAL_TOOL and OC_BOOT_SYSTEM are present a few lines above, if you expand upwards. (While the previous name OC_BOOT_EXTERNAL_SYSTEM was similar to both these, I believe the meaning is not similar.)

EDIT: i.e. OC_BOOT_SYSTEM is for system actions, which I take it 'do something' to the system (e.g. reset nvram), but probably 'do something' should be construed narrowly enough to avoid including 'boot an OS'. For the existing 'External' Tool and 'External' OS I think 'external' more or less means 'provided by the user rather than detected by OpenCore'. Whereas the new 'OC_BOOT_EXTERNAL_SYSTEM' basically means 'boot something, but don't use OpenCore to boot it', which is why I thought 'OC_BOOT_UNMANAGED' might be better, avoiding the multiple use of 'external' mentioned in OP above.