build/openfpgaloader: support args with '-'

many openfpgaloader args have a name with '-' as per normal convention.

This kwarg now works: file_type="raw"

litex/build/openfpgaloader.py

@@ -66,7 +66,7 @@ def flash(self, address, data_file, external=False, unprotect_flash=False, verif
 
         # Handle kwargs for specific, less common cases.
         for key, value in kwargs.items():
-            cmd.append(f"--{key}")
+            cmd.append(f"--{key.replace('_', '-')}")
             if value is not None:
                 cmd.append(str(value))