amdfwtool: Clean up the Makefile of amdfwtool

Add Makefile.inc to compliant with other tools.
Makefile is kept for building amdfwtool by typing make
in the folder.

Change-Id: I3688d93de4459f5f838955892086b4b9bf30a9b8
Signed-off-by: Zheng Bao <fishbaozi@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/45286
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin Roth <martinroth@google.com>
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>

util/amdfwtool/Makefile.inc

@@ -0,0 +1,12 @@
+# SPDX-License-Identifier: BSD-3-Clause
+
+amdfwtoolobj = amdfwtool.o
+
+AMDFWTOOLCFLAGS=-O2 -Wall -Wextra -Wshadow
+
+$(objutil)/amdfwtool/%.o: $(top)/util/amdfwtool/%.c # $(HEADER)
+	$(HOSTCC) $(AMDFWTOOLCFLAGS) $(HOSTCFLAGS) -DCONFIG_ROM_SIZE=$(CONFIG_ROM_SIZE) -c -o $@ $<
+
+$(objutil)/amdfwtool/amdfwtool: $(addprefix $(objutil)/amdfwtool/,$(amdfwtoolobj))
+	printf "   AMDFWTOOL\n"
+	$(HOSTCC) $(addprefix $(objutil)/amdfwtool/,$(amdfwtoolobj)) -o $@