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

@@ -1,16 +1,18 @@
-#*****************************************************************************
-#
-# All rights reserved.
-#
 # SPDX-License-Identifier: BSD-3-Clause
 
-#*****************************************************************************
 HOSTCC ?= cc
 
-amdfwtool_exe : amdfwtool.c
-	$(HOSTCC) amdfwtool.c -o amdfwtool
+SRC = amdfwtool.c
+OBJ = $(SRC:%.c=%.o)
+TARGET = amdfwtool
+CFLAGS=-O2 -Wall -Wextra -Wshadow
 
-amdfwtool : amdfwtool_exe
+
+$(TARGET): $(OBJ)
+	$(CC) $(OBJ) $(LDFLAGS) -o $@
+
+%.o: %.c $(HEADER)
+	$(CC) $(CFLAGS) -c -o $@ $<
 
 clean:
-	@rm -f amdfwtool.o amdfwtool amdfwtool.exe
+	@rm -f $(TARGET) $(OBJ)