diff --git a/hw/bsp/metro_m0_express/board.mk b/hw/bsp/metro_m0_express/board.mk
index 49b3d575..1844eeec 100644
--- a/hw/bsp/metro_m0_express/board.mk
+++ b/hw/bsp/metro_m0_express/board.mk
@@ -1,4 +1,4 @@
-CFLAGS = \
+CFLAGS += \
 	-DCFG_TUSB_MCU=OPT_MCU_SAMD21 \
 	-DCONF_DFLL_OVERWRITE_CALIBRATION=0 \
 	-D__SAMD21G18A__ \
diff --git a/hw/bsp/metro_m4_express/board.mk b/hw/bsp/metro_m4_express/board.mk
index 6e324c5a..a560c514 100644
--- a/hw/bsp/metro_m4_express/board.mk
+++ b/hw/bsp/metro_m4_express/board.mk
@@ -1,4 +1,4 @@
-CFLAGS = \
+CFLAGS += \
 	-DCFG_TUSB_MCU=OPT_MCU_SAMD51 \
 	-D__SAMD51J19A__ \
 	-mthumb \
@@ -8,6 +8,8 @@ CFLAGS = \
 	-mfpu=fpv4-sp-d16 \
 	-nostdlib
 
+CFLAGS += -Wno-error=undef
+
 # All source paths should be relative to the top level.
 LD_FILE = hw/bsp/metro_m4_express/samd51g19a_flash.ld
 
diff --git a/hw/bsp/pca10056/board.mk b/hw/bsp/pca10056/board.mk
index 402a295b..c60bf8be 100644
--- a/hw/bsp/pca10056/board.mk
+++ b/hw/bsp/pca10056/board.mk
@@ -7,6 +7,9 @@ CFLAGS += \
 	-mfloat-abi=hard \
 	-mfpu=fpv4-sp-d16
 
+# nrfx issue undef _ARMCC_VERSION usage https://github.com/NordicSemiconductor/nrfx/issues/49
+CFLAGS += -Wno-error=undef 
+
 # All source paths should be relative to the top level.
 LD_FILE = hw/mcu/nordic/nrfx/mdk/nrf52840_xxaa.ld
 
diff --git a/hw/bsp/stm32f407g_disc1/board.mk b/hw/bsp/stm32f407g_disc1/board.mk
index d09dc5bc..73d64a6b 100644
--- a/hw/bsp/stm32f407g_disc1/board.mk
+++ b/hw/bsp/stm32f407g_disc1/board.mk
@@ -1,4 +1,4 @@
-CFLAGS = \
+CFLAGS += \
 	-DHSE_VALUE=8000000 \
 	-DCFG_TUSB_MCU=OPT_MCU_STM32F4 \
 	-DSTM32F407xx \