[stm32][bsp] fix stm32f769-st-disco bsp scons --dist function

bsp/stm32/stm32f769-st-disco/SConstruct

@@ -34,19 +34,27 @@ if rtconfig.PLATFORM == 'iar':
 Export('RTT_ROOT')
 Export('rtconfig')
 
+SDK_ROOT = os.path.abspath('./')
+
+if os.path.exists(SDK_ROOT + '/libraries'):
+    libraries_path_prefix = SDK_ROOT + '/libraries'
+else:
+    libraries_path_prefix = os.path.dirname(SDK_ROOT) + '/libraries'
+
+SDK_LIB = libraries_path_prefix
+Export('SDK_LIB')
+
 # prepare building environment
 objs = PrepareBuilding(env, RTT_ROOT, has_libcpu=False)
 
 stm32_library = 'STM32F7xx_HAL'
 rtconfig.BSP_LIBRARY_TYPE = stm32_library
 
-SDK_ROOT = os.path.abspath('./')
+# include libraries
+objs.extend(SConscript(os.path.join(libraries_path_prefix, stm32_library, 'SConscript')))
 
 # include drivers
-objs.extend(SConscript(os.path.dirname(SDK_ROOT) + '/libraries/STM32F7xx_HAL/SConscript'))
-
-# include libraries
-objs.extend(SConscript(os.path.dirname(SDK_ROOT) + '/libraries/HAL_Drivers/SConscript'))
+objs.extend(SConscript(os.path.join(libraries_path_prefix, 'HAL_Drivers', 'SConscript')))
 
 # make a building
 DoBuilding(TARGET, objs)