Fix invalid PEP 440 version string from nRF51 bootloader dirty bit

When the nRF51 bootloader reports a dirty build (bit 0x80 set in the
version data), cflib constructed a version string ending in bare "+",
e.g. "2024.1.0+". This is invalid PEP 440 and causes
packaging.version.Version() to raise an error during flashing.

This likely started failing when the packaging library tightened its
PEP 440 validation in a newer release.

Reproducible with any Crazyflie running custom-built nRF51 firmware
(which sets the dirty bit). Fix changes the suffix from "+" to "+dirty"
which is valid PEP 440.

Tested by flashing a Crazyflie with dirty nRF51 bootloader firmware.

Author: gemenerik

cflib/bootloader/cloader.py

@@ -204,7 +204,7 @@ def _update_info(self, target_id):
                 if len(answer.data) > 23 and len(answer.data) > 26:
                     code_state = ''
                     if answer.data[24] & 0x80 != 0:
-                        code_state = '+'
+                        code_state = '+dirty'
                     answer.data[24] &= 0x7F
                     major = struct.unpack('H', answer.data[23:25])[0]
                     minor = answer.data[25]