Load optional modules dropped into src/modules/optional/ (#11673)

* Load optional modules dropped into src/modules/optional/

bin/optional-modules.py scans src/modules/optional/ for a directory <Name>/ holding <Name>.h and generates $BUILD_DIR/OptionalModules.h with an include and a setup<Name>() call for each, which Modules.cpp picks up through __has_include. The directory does not exist in a stock checkout, so a stock build generates a header that defines nothing, OPTIONAL_MODULES_SETUP compiles away, and nothing is registered. Sources under the directory are already covered by the default recursive build_src_filter, so dropping a module in needs no platformio.ini edit.

* Address review: skip a module directory that is not a usable identifier

The directory name becomes a setup<Name>() call, so foo-bar/ would have generated setupfoo-bar() and failed to compile with the error pointing at generated code rather than at the directory. Names that cannot form an identifier are now skipped with a message that names the directory.

src/modules/Modules.cpp

@@ -34,6 +34,11 @@
 #if !MESHTASTIC_EXCLUDE_BEACON
 #include "modules/MeshBeaconModule.h"
 #endif
+// Generated by bin/optional-modules.py from whatever sits in src/modules/optional/. Absent from a
+// stock checkout, and empty unless a module has been dropped in.
+#if __has_include("OptionalModules.h")
+#include "OptionalModules.h"
+#endif
 #if !MESHTASTIC_EXCLUDE_GPS
 #include "modules/PositionModule.h"
 #endif
@@ -283,6 +288,11 @@ void setupModules()
 #endif
 #if defined(HAS_HARDWARE_WATCHDOG)
     watchdogThread = new WatchdogThread();
+#endif
+    // Anything dropped into src/modules/optional/. Undefined, so compiled away, unless a module is
+    // actually present.
+#ifdef OPTIONAL_MODULES_SETUP
+    OPTIONAL_MODULES_SETUP();
 #endif
     // NOTE! This module must be added LAST because it likes to check for replies from other modules and avoid sending extra
     // acks