Allow setting values from base64 values

By prefixing a value with "base64:" e.g. ("base64:AQ==") it will now be
decoded to raw bytes. This is useful when setting psk as that is often
shown as base64.

meshtastic/util.py

@@ -64,6 +64,8 @@ def fromStr(valstr):
     elif valstr.startswith('0x'):
         # if needed convert to string with asBytes.decode('utf-8')
         val = bytes.fromhex(valstr[2:])
+    elif valstr.startswith('base64:'):
+        val = base64.b64decode(valstr[7:])
     elif valstr.lower() in {"t", "true", "yes"}:
         val = True
     elif valstr.lower() in {"f", "false", "no"}: