kbuild: Add generic rule to apply fdtoverlay

Add a generic rule to apply fdtoverlay in Makefile.lib, so every
platform doesn't need to carry the complex rule. This also automatically
adds "DTC_FLAGS_foo_base += -@" for all base files.

The platform's Makefile only needs to have this now:

 foo-dtbs := foo_base.dtb foo_overlay1.dtbo foo_overlay2.dtbo
 dtb-y := foo.dtb

We don't want to run schema checks on foo.dtb (as foo.dts doesn't exist)
and the Makefile is updated accordingly.
Acked-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Co-developed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Frank Rowand <frank.rowand@sony.com>
Tested-by: Frank Rowand <frank.rowand@sony.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20920b0df6b067aca4040459a9677d7d1d6d766a.1615354376.git.viresh.kumar@linaro.org

scripts/Makefile.lib

@@ -75,11 +75,24 @@ always-y += $(userprogs-always-y) $(userprogs-always-m)
 # If CONFIG_OF_ALL_DTBS is enabled, all DT blobs are built
 dtb-$(CONFIG_OF_ALL_DTBS)       += $(dtb-)
 
+# List all dtbs to be generated by fdtoverlay
+overlay-y := $(foreach m,$(dtb-y), $(if $(strip $($(m:.dtb=-dtbs))),$(m),))
+
+# Generate symbols for the base files so overlays can be applied to them.
+$(foreach m,$(overlay-y), $(eval DTC_FLAGS_$(basename $(firstword $($(m:.dtb=-dtbs)))) += -@))
+
+# Add base dtb and overlay dtbo
+dtb-y += $(foreach m,$(overlay-y), $($(m:.dtb=-dtbs)))
+
 always-y			+= $(dtb-y)
 
 ifneq ($(CHECK_DTBS),)
-always-y += $(patsubst %.dtb,%.dt.yaml, $(dtb-y))
-always-y += $(patsubst %.dtbo,%.dt.yaml, $(dtb-y))
+# Don't run schema checks for dtbs created by fdtoverlay as they don't
+# have corresponding dts files.
+dt-yaml-y := $(filter-out $(overlay-y),$(dtb-y))
+
+always-y += $(patsubst %.dtb,%.dt.yaml, $(dt-yaml-y))
+always-y += $(patsubst %.dtbo,%.dt.yaml, $(dt-yaml-y))
 endif
 
 # Add subdir path
@@ -337,6 +350,15 @@ $(obj)/%.dtb: $(src)/%.dts $(DTC) FORCE
 $(obj)/%.dtbo: $(src)/%.dts $(DTC) FORCE
 	$(call if_changed_dep,dtc)
 
+overlay-y := $(addprefix $(obj)/, $(overlay-y))
+
+quiet_cmd_fdtoverlay = DTOVL   $@
+      cmd_fdtoverlay = $(objtree)/scripts/dtc/fdtoverlay -o $@ -i $(real-prereqs)
+
+$(overlay-y): FORCE
+	$(call if_changed,fdtoverlay)
+$(call multi_depend, $(overlay-y), .dtb, -dtbs)
+
 DT_CHECKER ?= dt-validate
 DT_CHECKER_FLAGS ?= $(if $(DT_SCHEMA_FILES),,-m)
 DT_BINDING_DIR := Documentation/devicetree/bindings