diff --git a/ports/stm32/mboot/Makefile b/ports/stm32/mboot/Makefile
index 79aa3316f9..a72b752ac9 100755
--- a/ports/stm32/mboot/Makefile
+++ b/ports/stm32/mboot/Makefile
@@ -57,6 +57,10 @@ CFLAGS_MCU_h7 = $(CFLAGS_CORTEX_M) -mtune=cortex-m7 -mcpu=cortex-m7
 CFLAGS_MCU_l4 = $(CFLAGS_CORTEX_M) -mtune=cortex-m4 -mcpu=cortex-m4
 CFLAGS_MCU_wb = $(CFLAGS_CORTEX_M) -mtune=cortex-m4 -mcpu=cortex-m4
 
+# Standard C functions like memset need to be compiled with special flags so
+# the compiler does not optimise these functions in terms of themselves.
+CFLAGS_BUILTIN ?= -ffreestanding -fno-builtin -fno-lto
+
 CFLAGS = $(INC) -Wall -Wpointer-arith -Wdouble-promotion -Wfloat-conversion -Werror -std=gnu99 -nostdlib $(CFLAGS_MOD) $(CFLAGS_EXTRA)
 CFLAGS += -D$(CMSIS_MCU)
 CFLAGS += $(CFLAGS_MCU_$(MCU_SERIES))
@@ -84,6 +88,7 @@ else
 COPT += -Os -DNDEBUG
 endif
 
+$(BUILD)/lib/libc/string0.o: CFLAGS += $(CFLAGS_BUILTIN)
 LIB_SRC_C = \
 	lib/libc/string0.c \
 	lib/oofatfs/ff.c \