Revert "Remove -nostartfiles linker option from bootloader (#5996)"

This reverts commit 17b4aa2466.

Removing the -nostartfiles caused the Taranis bootloader to include
a number of strange functions. Instead define _fini to avoid the linker
issues.

radio/src/targets/common/arm/stm32/bootloader/CMakeLists.txt

@@ -73,6 +73,7 @@ set(BOOTLOADER_SRC
   ../usb_bsp.c
   ../usb_driver.cpp
   ../flash_driver.cpp
+  init.c
   boot.cpp
   bin_files.cpp
   )
@@ -111,7 +112,7 @@ remove_definitions(-DCLI)
 remove_definitions(-DUSB_SERIAL)
 add_definitions(-DBOOT)
 
-set(CMAKE_EXE_LINKER_FLAGS "-mcpu=${MCU} -mthumb -lm -T${RADIO_SRC_DIRECTORY}/targets/${TARGET_DIR}/stm32_ramboot.ld -Wl,-Map=bootloader.map,--cref,--no-warn-mismatch,--gc-sections")
+set(CMAKE_EXE_LINKER_FLAGS "-mcpu=${MCU} -mthumb -nostartfiles -lm -T${RADIO_SRC_DIRECTORY}/targets/${TARGET_DIR}/stm32_ramboot.ld -Wl,-Map=bootloader.map,--cref,--no-warn-mismatch,--gc-sections")
 
 add_executable(bootloader ${BOOTLOADER_SRC})
 add_dependencies(bootloader ${BITMAPS_TARGET} firmware_translations)

radio/src/targets/common/arm/stm32/bootloader/boot.cpp

@@ -526,4 +526,8 @@ int main()
   }
 
   return 0;
-}
+}
+
+#if defined(PCBHORUS)
+void *__dso_handle = 0;
+#endif

radio/src/targets/common/arm/stm32/bootloader/init.c

@@ -0,0 +1,9 @@
+void _init (void)
+{
+
+}
+
+void _fini(void)
+{
+
+}