'nil' means the 'first kernel found in module_path', which is the same
interpretation as passing 'nil' to loadkernel.
Otherwise, it denotes the name of a kernel that we've successfully loaded.
When reloaded later, we will still need to do the full search again to
locate the actual kernel in case things have changed, so just the name is
good enough.
This is in support of upcoming boot environment support. vfs.root.mountfrom
and currdev will be changed, then we will reload configuration and attempt
to reload the currently chosen kernel unless we shouldn't.
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