don't overwrite existing platformio build flags in build script

build.sh

@@ -47,8 +47,8 @@ build_firmware() {
   # e.g: RAK_4631_Repeater-v1.0.0-SHA
   FIRMWARE_FILENAME="$1-${FIRMWARE_VERSION_STRING}"
 
-  # export build flags for pio so we can inject firmware version info
+  # add firmware version info to end of existing platformio build flags in environment vars
-  export PLATFORMIO_BUILD_FLAGS="-DFIRMWARE_BUILD_DATE='\"${FIRMWARE_BUILD_DATE}\"' -DFIRMWARE_VERSION='\"${FIRMWARE_VERSION_STRING}\"'"
+  export PLATFORMIO_BUILD_FLAGS="${PLATFORMIO_BUILD_FLAGS} -DFIRMWARE_BUILD_DATE='\"${FIRMWARE_BUILD_DATE}\"' -DFIRMWARE_VERSION='\"${FIRMWARE_VERSION_STRING}\"'"
 
   # build firmware target
   pio run -e $1