systemd-boot-cfg.bbclass: Don't reference or set OVERRIDES
There's no need to add to the local copy of overrides and then not do
anything with it.
Now that this function is being used in package creation it was causing
sstate issues as well, as MACHINE is always in OVERRIDES, so something
trivial such as the name of the MACHINE would cause the hash to change.
Signed-off-by: California Sullivan <california.l.sullivan@intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
