kbuild: avoid build error when single DTB is turned into composite DTB

As commit afa974b7 ("kbuild: add real-prereqs shorthand for
$(filter-out FORCE,$^)") explained, $(real-prereqs) is not just a list
of objects when linking a multi-object module. If a single-object module
is turned into a multi-object module, $^ (and therefore $(real-prereqs)
as well) contains header files recorded in the *.cmd file. Such headers
must be filtered out.

Now that a DTB can be built either from a single source or multiple
source files, the same issue can occur.

Consider the following scenario:

First, foo.dtb is implemented as a single-blob device tree.

The code looks something like this:

[Sample Code 1]

  Makefile:

      dtb-y += foo.dtb

  foo.dts:

    #include <dt-bindings/gpio/gpio.h>
    /dts-v1/;
    / { };

When it is compiled, .foo.dtb.cmd records that foo.dtb depends on
scripts/dtc/include-prefixes/dt-bindings/gpio/gpio.h.

Later, foo.dtb is split into a base and an overlay. The code looks
something like this:

[Sample Code 2]

  Makefile:

      dtb-y += foo.dtb
      foo-dtbs := foo-base.dtb foo-addon.dtbo

  foo-base.dts:

    #include <dt-bindings/gpio/gpio.h>
    /dts-v1/;
    / { };

  foo-addon.dtso:

    /dts-v1/;
    /plugin/;
    / { };

If you rebuild foo.dtb without 'make clean', you will get this error:

    Overlay 'scripts/dtc/include-prefixes/dt-bindings/gpio/gpio.h' is incomplete

$(real-prereqs) contains not only foo-base.dtb and foo-addon.dtbo but
also scripts/dtc/include-prefixes/dt-bindings/gpio/gpio.h, which is
passed to scripts/dtc/fdtoverlay.

Fixes: 15d16d6d ("kbuild: Add generic rule to apply fdtoverlay")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

scripts/Makefile.lib

@@ -407,8 +407,12 @@ cmd_dtc = $(HOSTCC) -E $(dtc_cpp_flags) -x assembler-with-cpp -o $(dtc-tmp) $< ;
 		-d $(depfile).dtc.tmp $(dtc-tmp) ; \
 	cat $(depfile).pre.tmp $(depfile).dtc.tmp > $(depfile)
 
+# NOTE:
+# Do not replace $(filter %.dtb %.dtbo, $^) with $(real-prereqs). When a single
+# DTB is turned into a multi-blob DTB, $^ will contain header file dependencies
+# recorded in the .*.cmd file.
 quiet_cmd_fdtoverlay = DTOVL   $@
-      cmd_fdtoverlay = $(objtree)/scripts/dtc/fdtoverlay -o $@ -i $(real-prereqs)
+      cmd_fdtoverlay = $(objtree)/scripts/dtc/fdtoverlay -o $@ -i $(filter %.dtb %.dtbo, $^)
 
 $(multi-dtb-y): FORCE
 	$(call if_changed,fdtoverlay)