arch/stm32: Only compile stm32_serial.c when STM32_USART is enabled

When SEMIHOST_SYSLOG is enabled it creates its own up_putc() function
however the stm32_serial.c also creates this same function, even
when all STM32_USARTs are disabled. Fix patch fixes this issue.

Signed-off-by: Alan C. Assis <acassis@gmail.com>

arch/arm/src/stm32/CMakeLists.txt

@@ -35,7 +35,6 @@ list(
   stm32_flash.c
   stm32_irq.c
   stm32_lowputc.c
-  stm32_serial.c
   stm32_spi.c
   stm32_i2s.c
   stm32_sdio.c
@@ -46,6 +45,10 @@ list(
   stm32_capture.c
   stm32_dfumode.c)
 
+if(CONFIG_STM32_USART)
+  list(APPEND SRCS stm32_serial.c)
+endif()
+
 if(CONFIG_STM32_DMA)
   list(APPEND SRCS stm32_dma.c)
 endif()

arch/arm/src/stm32/Make.defs

@@ -25,10 +25,14 @@ include armv7-m/Make.defs
 CHIP_CSRCS  = stm32_allocateheap.c stm32_start.c stm32_rcc.c stm32_lse.c
 CHIP_CSRCS += stm32_lsi.c stm32_gpio.c stm32_exti_gpio.c stm32_flash.c
 CHIP_CSRCS += stm32_irq.c stm32_lowputc.c
-CHIP_CSRCS += stm32_serial.c stm32_spi.c stm32_i2s.c stm32_sdio.c stm32_tim.c
+CHIP_CSRCS += stm32_spi.c stm32_i2s.c stm32_sdio.c stm32_tim.c
 CHIP_CSRCS += stm32_waste.c stm32_ccm.c stm32_uid.c stm32_capture.c
 CHIP_CSRCS += stm32_dfumode.c
 
+ifeq ($(CONFIG_STM32_USART),y)
+CHIP_CSRCS += stm32_serial.c
+endif
+
 ifeq ($(CONFIG_STM32_DMA),y)
 CHIP_CSRCS += stm32_dma.c
 endif