runqemu-export-rootfs: fix inconsistent var names

runqemu-export-rootfs: fix inconsistent var names

Fixed:
$ runqemu nfs qemux86-64
[snip]
On your target please remember to add the following options for NFS
nfsroot=IP_ADDRESS:/path/to/nfsroot,nfsvers=3,port=,mountprog=,nfsprog=,udp,mountport=
[snip]

Note that the values are null, this is because their var names are
inconsistent.

[YOCTO #10519]

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Armin Kuster <akuster808@gmail.com>