Fix Linux Kernel fragment for netboot

Fix Linux Kernel fragment for netboot

Previous change caused issue of missing nbd.cfg at bitbaking for non-netboot environment.
This patch fix netboot setting.
(*)https://gerrit.automotivelinux.org/gerrit/#/c/12063/

Change-Id: I61a9e2ac9aecc98b6a379c4587a025710aaebfbb
Signed-off-by: Kotaro Hashimoto <Hashimoto.Kotaro@ds.MitsubishiElectric.co.jp>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/12217
Reviewed-by: Thomas Rini <trini@konsulko.com>
Reviewed-by: Stéphane Desneux <stephane.desneux@iot.bzh>
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Tested-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>