boot: don't pass kernel cmdline option to UKIs which have the very same line built-in

boot: don't pass kernel cmdline option to UKIs which have the very same line built-in

There's really no point in first extracting a command line from an UKI
to just pass it unmodified to the UKI. In particular as this is
systematically ignored in SecureBoot is enabled.

Let's drop this, so that unless the user actually edits the cmdline we
pass nothing to the invoked kernel.

Note that this low-key is a compat break, since the passed cmdline gets
measured. However, in the interest of minimized the difference between
systems with and without sd-boot to behave the same i think we should
suppress the command line anyway.