meta-netboot: enhance reference uboot config for R-Car Gen3

meta-netboot: enhance reference uboot config for R-Car Gen3

The provided config now allows to switch from netboot to sdcard
boot at runtime by changing a uboot variable.

Change-Id: I778f46f65ca007dbdd5ae070e946c04b7277ddeb
Signed-off-by: Stephane Desneux <stephane.desneux@iot.bzh>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/12213
Reviewed-by: Thomas Rini <trini@konsulko.com>
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Tested-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>