net: wan: wanxl: refactor the firmware rebuild rule

Split the big recipe into 3 stages: compile, link, and hexdump.

After this commit, the build log with CONFIG_WANXL_BUILD_FIRMWARE
will look like this:

  M68KAS  drivers/net/wan/wanxlfw.o
  M68KLD  drivers/net/wan/wanxlfw.bin
  BLDFW   drivers/net/wan/wanxlfw.inc
  CC [M]  drivers/net/wan/wanxl.o
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

drivers/net/wan/Makefile

@@ -47,14 +47,23 @@ else
   M68KLD = $(CROSS_COMPILE_M68K)ld
 endif
 
-quiet_cmd_build_wanxlfw = BLD FW  $@
-      cmd_build_wanxlfw = \
-	$(M68KCC) -D__ASSEMBLY__ -Wp,-MD,$(depfile) -I$(srctree)/include/uapi -c -o $(obj)/wanxlfw.o $<; \
-	$(M68KLD) --oformat binary -Ttext 0x1000 $(obj)/wanxlfw.o -o $(obj)/wanxlfw.bin; \
-	hexdump -ve '"\n" 16/1 "0x%02X,"' $(obj)/wanxlfw.bin | sed 's/0x  ,//g;1s/^/static const u8 firmware[]={/;$$s/,$$/\n};\n/' >$(obj)/wanxlfw.inc; \
-	rm -f $(obj)/wanxlfw.bin $(obj)/wanxlfw.o
-
-$(obj)/wanxlfw.inc:	$(src)/wanxlfw.S
-	$(call if_changed_dep,build_wanxlfw)
-targets += wanxlfw.inc
+quiet_cmd_build_wanxlfw = BLDFW   $@
+      cmd_build_wanxlfw = hexdump -ve '"\n" 16/1 "0x%02X,"' $< | \
+	sed 's/0x  ,//g;1s/^/static const u8 firmware[]={/;$$s/,$$/\n};\n/' > $@
+
+$(obj)/wanxlfw.inc: $(obj)/wanxlfw.bin FORCE
+	$(call if_changed,build_wanxlfw)
+
+quiet_cmd_m68kld_bin_o = M68KLD  $@
+      cmd_m68kld_bin_o = $(M68KLD) --oformat binary -Ttext 0x1000 $< -o $@
+
+$(obj)/wanxlfw.bin: $(obj)/wanxlfw.o FORCE
+	$(call if_changed,m68kld_bin_o)
+
+quiet_cmd_m68kas_o_S = M68KAS  $@
+      cmd_m68kas_o_S = $(M68KCC) -D__ASSEMBLY__ -Wp,-MD,$(depfile) -I$(srctree)/include/uapi -c -o $@ $<
+
+$(obj)/wanxlfw.o: $(src)/wanxlfw.S FORCE
+	$(call if_changed_dep,m68kas_o_S)
 endif
+targets += wanxlfw.inc wanxlfw.bin wanxlfw.o