firmware: Create directories for external firmware

Commit 5180d5f4 ("firmware: Simplify directory creation") broke
including firmware specified in CONFIG_EXTRA_FIRMWARE:

  MK_FW   firmware/amd-ucode/microcode_amd.bin.gen.S
/bin/sh: firmware/amd-ucode/microcode_amd.bin.gen.S: No such file or directory
...
firmware/Makefile:185: recipe for target
'firmware/amd-ucode/microcode_amd.bin.gen.S' failed

It works with O= builds, because the directory is created by
Makefile.build. Create the directory in firmware/Makefile in non-O
builds.
Reported-and-tested-by: NRonald Uitermark <ronald645@gmail.com>
Reported-and-tested-by: NTorsten Kaiser <just.for.lkml@googlemail.com>
Signed-off-by: NMichal Marek <mmarek@suse.cz>

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)/%,%, \