[MIPS] MIPSsim: Fix booting from NFS root

MIPSsim probably doesn't have any sort of environment, but writing
a zero in it kills even the compiled in command line. This prevents
booting via NFS root.

Signed-Off-By: Thiemo Seufer <ths@networkno.de>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>