scripts: dtc: Build fdtoverlay tool

We will start building overlays for platforms soon in the kernel and
would need fdtoverlay going forward. Lets start building it.

The fdtoverlay program applies one or more overlay dtb blobs to a base
dtb blob. The kernel build system would later use fdtoverlay to generate
the overlaid blobs based on platform specific configurations.
Signed-off-by: NViresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: NRob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/4a201dea3ba11a00cab7e936dfc1140dac1a1ae3.1611904394.git.viresh.kumar@linaro.org

scripts/dtc/Makefile

 # SPDX-License-Identifier: GPL-2.0
 # scripts/dtc makefile
 
-hostprogs-always-$(CONFIG_DTC)		+= dtc
+hostprogs-always-$(CONFIG_DTC)		+= dtc fdtoverlay
 hostprogs-always-$(CHECK_DT_BINDING)	+= dtc
 
 dtc-objs	:= dtc.o flattree.o fstree.o data.o livetree.o treesource.o \
 		   srcpos.o checks.o util.o
 dtc-objs	+= dtc-lexer.lex.o dtc-parser.tab.o
 
+# The upstream project builds libfdt as a separate library.  We are choosing to
+# instead directly link the libfdt object files into fdtoverlay.
+libfdt-objs	:= fdt.o fdt_ro.o fdt_wip.o fdt_sw.o fdt_rw.o fdt_strerror.o fdt_empty_tree.o fdt_addresses.o fdt_overlay.o
+libfdt		= $(addprefix libfdt/,$(libfdt-objs))
+fdtoverlay-objs	:= $(libfdt) fdtoverlay.o util.o
+
 # Source files need to get at the userspace version of libfdt_env.h to compile
 HOST_EXTRACFLAGS += -I $(srctree)/$(src)/libfdt