minikconf: do not include variables from MINIKCONF_ARGS in config-all-devices.mak

minikconf: do not include variables from MINIKCONF_ARGS in config-all-devices.mak

When minikconf writes config-devices.mak, it includes all variables including
those from MINIKCONF_ARGS.  This causes values from config-host.mak to "stick" to
the ones used in generating config-devices.mak, because config-devices.mak is
included after config-host.mak.  Avoid this by omitting assignments coming
from the command line in the output of minikconf.

Reported-by: Christophe de Dinechin <dinechin@redhat.com>
Reviewed-by: Christophe de Dinechin <dinechin@redhat.com>
Tested-by: Christophe de Dinechin <dinechin@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>