scripts: ci: check_compliance.py: Fix check_disallowed_defconfigs: Windows

#78615 Added a check for disallowed Kconfigs which does not work
on Windows machines. Windows machines seem to behave differently when
faced with the \b word boundary marker. I have removed the \b word
boundary marker from the grep commands when os.name=='nt'.
The script performs the same as before for any other os.

Signed-off-by: Jacob Wienecke <jacob.wienecke@nxp.com>