linux: add support for device tree overlays

The linux kernel can build device tree overlays (.dtbo) itself. Add
support to build and copy them along with the actual device trees.
These can either be in-tree device tree overlays
(BR2_LINUX_KERNEL_INTREE_DTSO_NAMES) or they can be provided outside of
the kernel (BR2_LINUX_KERNEL_CUSTOM_DTS_PATH). In the latter case, the
overlay source files will be copied into the kernel tree first.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Niklas Cassel <niklas.cassel@wdc.com>
Signed-off-by: Romain Naour <romain.naour@smile.fr>
Signed-off-by: Julien Olivain <ju.o@free.fr>

linux/Config.in

@@ -426,12 +426,20 @@ config BR2_LINUX_KERNEL_INTREE_DTS_NAME
 	  the trailing .dts. You can provide a list of
 	  dts files to build, separated by spaces.
 
+config BR2_LINUX_KERNEL_INTREE_DTSO_NAMES
+	string "In-tree Device Tree Overlay file names"
+	help
+	  Names of in-tree device tree overlay, without the trailing
+	  .dtso which should be built and installed into the target
+	  system, separated by spaces.
+
 config BR2_LINUX_KERNEL_CUSTOM_DTS_PATH
 	string "Out-of-tree Device Tree Source file paths"
 	help
-	  Paths to out-of-tree Device Tree Source (.dts) and Device Tree
-	  Source Include (.dtsi) files, separated by spaces. These files
-	  will be copied to the kernel sources and the .dts files will
+	  Paths to out-of-tree Device Tree Source (.dts), Device Tree
+	  Source Include (.dtsi) and Device Tree Overlay Source (.dtso)
+	  files, separated by spaces. These files will be copied to the
+	  kernel sources and the .dts files will
 	  be compiled from there.
 
 config BR2_LINUX_KERNEL_DTB_KEEP_DIRNAME

linux/linux.mk

@@ -203,14 +203,17 @@ endif
 LINUX_VERSION_PROBED = `MAKEFLAGS='$(filter-out w,$(MAKEFLAGS))' $(BR2_MAKE) $(LINUX_MAKE_FLAGS) -C $(LINUX_DIR) --no-print-directory -s kernelrelease 2>/dev/null`
 
 LINUX_DTS_NAME += $(call qstrip,$(BR2_LINUX_KERNEL_INTREE_DTS_NAME))
+LINUX_DTSO_NAMES += $(call qstrip,$(BR2_LINUX_KERNEL_INTREE_DTSO_NAMES))
 
 # We keep only the .dts files, so that the user can specify both .dts
 # and .dtsi files in BR2_LINUX_KERNEL_CUSTOM_DTS_PATH. Both will be
 # copied to arch/<arch>/boot/dts, but only the .dts files will
 # actually be generated as .dtb.
-LINUX_DTS_NAME += $(basename $(filter %.dts,$(notdir $(call qstrip,$(BR2_LINUX_KERNEL_CUSTOM_DTS_PATH)))))
+LINUX_CUSTOM_DTS_PATH = $(call qstrip,$(BR2_LINUX_KERNEL_CUSTOM_DTS_PATH))
+LINUX_DTS_NAME += $(basename $(filter %.dts,$(notdir $(LINUX_CUSTOM_DTS_PATH))))
+LINUX_DTSO_NAMES += $(basename $(filter %.dtso,$(notdir $(LINUX_CUSTOM_DTS_PATH))))
 
-LINUX_DTBS = $(addsuffix .dtb,$(LINUX_DTS_NAME))
+LINUX_DTBS = $(addsuffix .dtb,$(LINUX_DTS_NAME)) $(addsuffix .dtbo,$(LINUX_DTSO_NAMES))
 
 ifeq ($(BR2_LINUX_KERNEL_IMAGE_TARGET_CUSTOM),y)
 LINUX_IMAGE_NAME = $(call qstrip,$(BR2_LINUX_KERNEL_IMAGE_NAME))