buildroot

support/testing: use pexpect in emulator

Replace subprocess + telnetlib with pexpect.

Use the telnet installed on the host machine instead of telnetlib, while
the serial from qemu is not yet redirected to stdio.

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>