fs: convert makefiles to Kbuild style

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/fs/fat/Makefile b/fs/fat/Makefile
index c00681f..b60e848 100644
--- a/fs/fat/Makefile
+++ b/fs/fat/Makefile
@@ -3,34 +3,9 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
-include $(TOPDIR)/config.mk
-
-LIB	= $(obj)libfat.o
-
-AOBJS	=
-COBJS-$(CONFIG_FS_FAT)	:= fat.o
-COBJS-$(CONFIG_FAT_WRITE):= fat_write.o
+obj-$(CONFIG_FS_FAT)	:= fat.o
+obj-$(CONFIG_FAT_WRITE):= fat_write.o
 
 ifndef CONFIG_SPL_BUILD
-COBJS-$(CONFIG_FS_FAT)	+= file.o
+obj-$(CONFIG_FS_FAT)	+= file.o
 endif
-
-SRCS	:= $(AOBJS:.o=.S) $(COBJS-y:.o=.c)
-OBJS	:= $(addprefix $(obj),$(AOBJS) $(COBJS-y))
-
-all:	$(LIB) $(AOBJS)
-
-$(LIB):	$(obj).depend $(OBJS)
-	$(call cmd_link_o_target, $(OBJS))
-
-# SEE README.arm-unaligned-accesses
-$(obj)file.o: CFLAGS += $(PLATFORM_NO_UNALIGNED)
-
-#########################################################################
-
-# defines $(obj).depend target
-include $(SRCTREE)/rules.mk
-
-sinclude $(obj).depend
-
-#########################################################################