Add build timestamp to the version number (#1145)

I've found this rather useful when repeatedly deploying updated firmware
images to a large number of nodes.

The patch carefully avoids full rebuilds of any "real" source files by
putting the timestamp into a separate .cpp file. It also works around
PlatformIO's build cache that would happily permanently cache the timestamp
file if we were using __DATE__.

The Python script isn't entirely necessary because we could just simply
touch the timestamp file via extra_scripts to force a rebuild. But we'd
need _some_ kind of script anyway and this way we can format the timestamp
more easily than is possible with C++.

.gitignore

@@ -14,3 +14,4 @@ CMakeLists.txt
 CMakeListsPrivate.txt
 platformio_override.ini
 node_modules
+src/build_timestamp_value.h

platformio.ini

@@ -61,6 +61,7 @@ board_build.partitions = partitions_singleapp.csv
 monitor_speed = 115200
 monitor_filters = esp32_exception_decoder, time
 upload_speed = 1500000
+extra_scripts = update_ts.py
 
 [esp32]
 extends = common

src/build_timestamp.cpp

@@ -0,0 +1,6 @@
+#include "build_timestamp.h"
+#include "build_timestamp_value.h"
+
+const char *getBuildTimestamp() {
+    return BUILD_TIMESTAMP;
+}

src/build_timestamp.h

@@ -0,0 +1 @@
+const char *getBuildTimestamp();

src/main.cpp

@@ -91,7 +91,7 @@ bool sendTelemetry(unsigned int totalSeen, unsigned int totalFpSeen, unsigned in
 #ifdef VERSION
     doc["ver"] = String(VERSION);
 #else
-    doc["ver"] = ESP.getSketchMD5();
+    doc["ver"] = ESP.getSketchMD5() + "-" + getBuildTimestamp();
 #endif
 
     if (!BleFingerprintCollection::countIds.isEmpty())

src/main.h

@@ -27,6 +27,7 @@
 #include "globals.h"
 #include "mqtt.h"
 #include "string_utils.h"
+#include "build_timestamp.h"
 #ifdef M5STICK
 #include <AXP192.h>
 #endif

update_ts.py

@@ -0,0 +1,6 @@
+#!/usr/bin/env python3
+from datetime import datetime
+
+with open("src/build_timestamp_value.h", "w") as fp:
+    timestamp = datetime.now().strftime("%Y-%m-%dT%H:%M:%S")
+    fp.write(f'#define BUILD_TIMESTAMP "{timestamp}"')