test: twister: hwmap: missing serial_pty definition

[hakehuang]

Describe the bug

the serial_pty definition is missing. in hardwaremap.py

Please also mention any information which could help others to understand
the problem you're facing:

• What target platform are you using?
• any
• What have you tried to diagnose or workaround this issue?
• No
• Is this a regression? If yes, have you been able to "git bisect" it to a
  specific commit?
• this seems missing cherry-pick 78f688a

To Reproduce

scripts/twister --device-testing --hardware-map map.yaml -T samples/

Expected behavior

Impact

Logs and console output

INFO    - Using Ninja..
INFO    - Zephyr version: v3.7.1-rc1
INFO    - Using 'zephyr' toolchain.
Traceback (most recent call last):
  File "/home/ubuntu/nxp/frdm_ke15z/zephyr/scripts/twister", line 215, in <module>
    ret = main(options, default_options)
  File "/home/ubuntu/nxp/frdm_ke15z/zephyr/scripts/pylib/twister/twisterlib/twister_main.py", line 115, in main
    ret = hwm.discover()
  File "/home/ubuntu/nxp/frdm_ke15z/zephyr/scripts/pylib/twister/twisterlib/hardwaremap.py", line 169, in discover
    self.load(self.options.hardware_map)
  File "/home/ubuntu/nxp/frdm_ke15z/zephyr/scripts/pylib/twister/twisterlib/hardwaremap.py", line 239, in load
    flash_before = self.options.flash_before and (not (flash_with_test or serial_pty))
UnboundLocalError: local variable 'serial_pty' referenced before assignment
Keeping artifacts untouched

Environment (please complete the following information):

• OS: (e.g. Linux, MacOS, Windows)
• Toolchain (e.g Zephyr SDK, ...)
• Commit SHA or Version used: v3.7.1-rc1

[hakehuang]

@nashif please check this, cherry-pick the 78f688a will fix this, and then I can run a board testing for this release, Thanks

[nashif]

merged