runqemu: add process of option QB_NFSROOTFS_EXTRA_OPT

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

Example config with qemuppc64 in machine config:

add r/w size in the nfs rootfs extra option:
QB_NFSROOTFS_EXTRA_OPT = "wsize=524288,rsize=524288"

re-build and runqemu with nfs again, we can observe the kernel command
line added our defined value in QB_NFSROOTFS_EXTRA_OPT:

Kernel command line: root=/dev/nfs nfsroot=10.0.2.2:/home/xchen5/testing/build/tmp-glibc/deploy/images/qemuppc64/testnfs,nfsvers=3,port=3049,tcp,mountport=3048,wsize=524288,rsize=524288 rw  mem=256M ip=dhcp console=hvc0 console=hvc0 nohugevmalloc

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