add IRQ demo for the STM32F070xB

fix core type for the STM32F070xB

issue #44

Makefile

@@ -188,4 +188,4 @@ stm32f070xb:
 	@$(MAKE) clean demo STARTUP='$(CMSISDEV)/ST/STM32F0xx/Source/Templates/gcc/startup_stm32f070xb.s' \
 						LDSCRIPT='demo/stm32f070xb.ld' \
 						DEFINES='STM32F0 STM32F070xB USBD_SOF_DISABLED' \
-						CFLAGS='-mcpu=cortex-m0plus -mthumb'
+						CFLAGS='-mcpu=cortex-m0 -mthumb'

demo/cdc_loop.c

@@ -464,7 +464,7 @@ static void cdc_init_usbd(void) {
 }
 
 #if defined(CDC_USE_IRQ)
-#if defined(STM32L052xx)
+#if defined(STM32L052xx) || defined(STM32F070xB)
     #define USB_HANDLER     USB_IRQHandler
     #define USB_NVIC_IRQ    USB_IRQn
 #elif defined(STM32L100xC)