firmware_build_epoch extern instead of BUILD_EPOCH macro

include/handlers.h

@@ -10,6 +10,7 @@ inline constexpr uint16_t PICOMAP_PORT_BE = __builtin_bswap16(28781);
 inline constexpr ipv4::ip4_addr PICOMAP_DISCOVERY_GROUP = {239, 112, 77, 1};
 
 extern std::string_view firmware_name;
+extern uint32_t firmware_build_epoch;
 
 void handlers_init();
 void handlers_start();

src/handlers.cpp

@@ -39,7 +39,7 @@ std::optional<ResponseInfo> handle_info(const responder&, const RequestInfo&) {
     resp.ip = ns.ip;
     resp.firmware_name = firmware_name;
     resp.boot = detected_boot_reason;
-    resp.build_epoch = BUILD_EPOCH;
+    resp.build_epoch = firmware_build_epoch;
     return resp;
 }