diff --git a/src/platforms/common/cdcacm.c b/src/platforms/common/cdcacm.c
index 36d3bd26..91136134 100644
--- a/src/platforms/common/cdcacm.c
+++ b/src/platforms/common/cdcacm.c
@@ -395,8 +395,8 @@ static const struct usb_config_descriptor config = {
 	.interface = ifaces,
 };
 
-#if defined(STM32L0) || defined(STM32F3) || defined(STM32F4)
-static char serial_no[13];
+#if defined(DUSE_ST_SERIAL)
+char serial_no[13];
 #else
 static char serial_no[9];
 #endif
diff --git a/src/platforms/f4discovery/Makefile.inc b/src/platforms/f4discovery/Makefile.inc
index 8596882b..c9116db5 100644
--- a/src/platforms/f4discovery/Makefile.inc
+++ b/src/platforms/f4discovery/Makefile.inc
@@ -17,9 +17,9 @@ LDFLAGS_BOOT = -lopencm3_stm32f4 \
 ifeq ($(BMP_BOOTLOADER), 1)
 $(info  Load address 0x08004000 for BMPBootloader)
 LDFLAGS = $(LDFLAGS_BOOT) -Wl,-Ttext=0x8004000
-CFLAGS += -DUSE_BMP_SERIAL
 else
-LDFLAGS = $(LDFLAGS_BOOT)
+LDFLAGS += $(LDFLAGS_BOOT)
+CFLAGS += -DUSE_ST_SERIAL
 endif
 
 VPATH += platforms/stm32
diff --git a/src/platforms/hydrabus/Makefile.inc b/src/platforms/hydrabus/Makefile.inc
index 24d43fa1..9d1d2a01 100644
--- a/src/platforms/hydrabus/Makefile.inc
+++ b/src/platforms/hydrabus/Makefile.inc
@@ -5,7 +5,7 @@ OBJCOPY = $(CROSS_COMPILE)objcopy
 CFLAGS += -Istm32/include -mcpu=cortex-m4 -mthumb \
 	-mfloat-abi=hard -mfpu=fpv4-sp-d16 \
 	-DSTM32F4 -DHYDRABUS -I../libopencm3/include \
-	-Iplatforms/stm32
+	-Iplatforms/stm32 -DUSE_ST_BOOTLOADER
 
 LDFLAGS = -lopencm3_stm32f4 \
 	-Wl,-T,platforms/stm32/f4discovery.ld -nostartfiles -lc -lnosys \