uboot-phytec

boot.src: Provide dsa_core.blacklist bootarg when booting via NFS

This setup is a bit special as Linux kernel (4.19) is not supporting
in-kernel NFS rootfs mount and Distributed Switch Architecture (DSA)
operation.

For that reason it was necessary to provide a bootarg to allow userspace
enable it afterwards if needed.

Signed-off-by: Lukasz Majewski <lukma@denx.de>