Fix startup assembly bugs in 500b

Root cause:
1. .s files are not processed by the preprocessor, since they are passed directly to the assembler.  Only .S files are processed by gcc, which uses the preprocessor before sending to the assembler.  Thus the #includes in startup_stdm32f.s had no effect.
2. The proxy_inc/**/*.s files were accidentally *not* filtered from the sources like the .c files in the same directory.  Thus the startup code for both boards was unconditionally included.
3. The two bugs cancelled each other out for the 500b, resulting in the correct startup code being built.  However, it was incorrectly including the 500b's startup code in the non-500b build.

stm32/ros_usbnode/platformio.ini

@@ -19,6 +19,7 @@ build_src_filter =
 	-<.git/>
 	-<.svn/>
 	-<proxy_inc/**/*.c>
+	-<proxy_inc/**/*.S>
 
 [env:Yardforce500]
 board = genericSTM32F103VC

stm32/ros_usbnode/src/startup_stm32f.S

@@ -0,0 +1,5 @@
+#if BOARD_YARDFORCE500_VARIANT_ORIG
+#include "proxy_inc/stm32f1/startup_stm32f1xx.S"
+#elif BOARD_YARDFORCE500_VARIANT_B
+#include "proxy_inc/stm32f4/startup_stm32f4xx.S"
+#endif