nfsrootfs: disable warning message if bootargs root parameter have not been defined

nfsrootfs: disable warning message if bootargs root parameter have not been defined

we have case that don't define root in bootargs,
if [ ${bootparam_root} != "/dev/nfs" ] will output warning "/init.d/85-nfsrootfs: line 4: [: !=: unary operator expected"
let variable expension result become string to solve this problem

Signed-off-by: Jimmy Ho <jimmy.ho@sifive.com>
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>