runners: nsim: allow to use runner if gdb is missing

As of today we check for gdb presence in 'do_create' method and
raise an exception in case of gdb is missing. That makes nsim
runner unusable without gdb even for the commands which don't use
it (like 'flash' command).

Fix that.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>

scripts/west_commands/runners/nsim.py

diff --git a/scripts/west_commands/runners/nsim.py b/scripts/west_commands/runners/nsim.py
index 002e2da..4ec1b76 100644
--- a/scripts/west_commands/runners/nsim.py
+++ b/scripts/west_commands/runners/nsim.py
@@ -28,7 +28,10 @@
                  gdb_port=DEFAULT_ARC_GDB_PORT,
                  props=DEFAULT_PROPS_FILE):
         super().__init__(cfg)
-        self.gdb_cmd = [cfg.gdb] + (['-tui'] if tui else [])
+        if cfg.gdb is None:
+            self.gdb_cmd = None
+        else:
+            self.gdb_cmd = [cfg.gdb] + (['-tui'] if tui else [])
         self.nsim_cmd = ['nsimdrv']
         self.gdb_port = gdb_port
         self.props = props
@@ -46,9 +49,6 @@
 
     @classmethod
     def do_create(cls, cfg, args):
-        if cfg.gdb is None:
-            raise ValueError('--gdb not provided at command line')
-
         return NsimBinaryRunner(
             cfg,
             gdb_port=args.gdb_port,
@@ -73,6 +73,9 @@
         self.check_call(cmd)
 
     def do_debug(self, **kwargs):
+        if self.gdb_cmd is None:
+            raise ValueError('Cannot debug; gdb is missing')
+
         config = kwargs['nsim-cfg']
 
         server_cmd = (self.nsim_cmd + ['-gdb',