bootctl: replace --no-variables by --variables=BOOL

bootctl: replace --no-variables by --variables=BOOL

I think the current behaviour of not doing EFI variables when we are run
in a container makes a ton of sense, but in some cases it's useful to
do EFI var setup even when a set of namespaces is set up for us, for
example to recover a hosed installation from a rescue disk.

While we are at it, let's remove some duplicate checks, and
systematically output information why we skip various operations.

Fixes: #36174 #35005