build: Build-Datum zusammen mit der Version bei jedem Build erzeugen

NINLIGHT_BUILD_DATE entstand als string(TIMESTAMP) zur Konfigurationszeit,
während die Version seit bf8f76e bei jedem Build neu geschrieben wird -
system/info.build_date konnte dadurch tagealt sein. Das Datum wandert in
cmake/write_version.cmake und damit in denselben generierten Header wie die
Version.

CMakeLists.txt

@@ -107,7 +107,6 @@ add_executable(${NL_FW} ${NL_SDK_BOOT_SOURCES} ${NL_FW_SOURCES} ${NL_SDK_OBJECTS
 set_target_properties(${NL_FW} PROPERTIES SUFFIX ".elf")
 target_compile_definitions(${NL_FW} PRIVATE
   NINLIGHT_DEVICE_ID="${NINLIGHT_DEVICE}"
-  NINLIGHT_BUILD_DATE="${NINLIGHT_BUILD_DATE}"
   NINLIGHT_TOKEN_DEFAULT="${NINLIGHT_TOKEN}"
   NINLIGHT_WIFI_SSID="${NINLIGHT_WIFI_SSID}"
   NINLIGHT_WIFI_PASSWORD="${NINLIGHT_WIFI_PASSWORD}"

cmake/version.cmake

@@ -1,8 +1,9 @@
-# The version is regenerated on every build so the OTA trailer and system/info match the binary.
+# Version *and* build date are regenerated on every build, so the OTA trailer and system/info
+# match the binary. The build date used to be a configure-time string(TIMESTAMP), which left
+# system/info.build_date days behind the actual build after the version generation moved here.
 set(NL_VERSION_HEADER ${CMAKE_BINARY_DIR}/generated/nl_version.h)
 add_custom_target(nl_version ALL
   BYPRODUCTS ${NL_VERSION_HEADER}
   COMMAND ${CMAKE_COMMAND} -DOUT=${NL_VERSION_HEADER} -DSRC=${CMAKE_SOURCE_DIR}
           -P ${CMAKE_SOURCE_DIR}/cmake/write_version.cmake
   COMMENT "Updating nl_version.h")
-string(TIMESTAMP NINLIGHT_BUILD_DATE "%Y-%m-%d")

cmake/write_version.cmake

@@ -5,7 +5,8 @@ if(NOT V)
 endif()
 string(REGEX REPLACE "^v" "" V "${V}")
 string(SUBSTRING "${V}" 0 15 V)
-set(CONTENT "#define NINLIGHT_FW_VERSION \"${V}\"\n")
+string(TIMESTAMP BUILD_DATE "%Y-%m-%d")
+set(CONTENT "#define NINLIGHT_FW_VERSION \"${V}\"\n#define NINLIGHT_BUILD_DATE \"${BUILD_DATE}\"\n")
 if(EXISTS ${OUT})
   file(READ ${OUT} OLD)
 else()