qemuboot.bbclass: add QB_NFSROOTFS_EXTRA_OPT for nfs rootfs extra option

This extra options to be appended to the nfs rootfs options in kernel boot arg.

Signed-off-by: Xiangyu Chen <xiangyu.chen@windriver.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>