oeqa/controllers/masterimage: add a serial control command

oeqa/controllers/masterimage: add a serial control command

Similar to power control command, this depends on a user's setup to get to
the serial port of a board. For a local connected board this could just be:

TEST_SERIALCONTROL_CMD = "picocom /dev/ttyUSB0 -b 115200"

and for a serial console server: "telnet 10.11.12.13 7003" or some conmux
command.

Signed-off-by: Stefan Stanacar <stefanx.stanacar@intel.com>
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>