diff --git a/firmware/Makefile b/firmware/Makefile
index 5747417069ca..0862d34cf7d1 100644
--- a/firmware/Makefile
+++ b/firmware/Makefile
@@ -219,6 +219,12 @@ $(obj)/%.fw: $(obj)/%.H16 $(ihex2fw_dep)
 obj-y				 += $(patsubst %,%.gen.o, $(fw-external-y))
 obj-$(CONFIG_FIRMWARE_IN_KERNEL) += $(patsubst %,%.gen.o, $(fw-shipped-y))
 
+ifeq ($(KBUILD_SRC),)
+# Makefile.build only creates subdirectories for O= builds, but external
+# firmware might live outside the kernel source tree
+_dummy := $(foreach d,$(addprefix $(obj)/,$(dir $(fw-external-y))), $(shell [ -d $(d) ] || mkdir -p $(d)))
+endif
+
 # Remove .S files and binaries created from ihex
 # (during 'make clean' .config isn't included so they're all in $(fw-shipped-))
 targets := $(fw-shipped-) $(patsubst $(obj)/%,%, \