Sim: fail to create the pipe file in the fifo directory

When the fifo directory do not be created at the first
time, the main of sim will fail to create. And other's
pipe file is fail. This will result the sim fail to
simulator the zigbee communication.

the error information in log/sim.txt as the following:

mknod: No such file or directory
mknod: No such file or directory
open public pipe: No such file or directory
read public pipe: Bad file descriptor
data_in open pipe: No such file or directory
cmd_in open pipe: No such file or directory
data_out open pipe: No such file or directory
cmd_out open pipe: No such file or directory
cmd read pipe: Bad file descriptor
read pipe: Bad file descriptor
read pipe: Bad file descriptor

signed-off-by: Zhong Hongbo <bocui107@gmail.com>