runqemu: work without SERIAL_CONSOLES being defined

Not all machine definitions need to have SERIAL_CONSOLES defined, but
runqemu currently will fail with the following script error if not
present:

Traceback (most recent call last):
  File "/builder/meta-arm/poky/scripts/runqemu", line 1604, in main
    config.setup_final()
  File "/builder/meta-arm/poky/scripts/runqemu", line 1446, in setup_final
    self.setup_serial()
  File "/builder/meta-arm/poky/scripts/runqemu", line 1381, in setup_serial
    self.kernel_cmdline_script += ' console=%s' %entry.split(';')[1]
IndexError: list index out of range

To get around this issue, add a sanity check to runqemu to avoid the
parsing of SERIAL_CONSOLES if empty.

Signed-off-by: Jon Mason <jdmason@kudzu.us>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>