cris: Only build flash rescue image if CONFIG_ETRAX_AXISFLASHMAP is selected

If CONFIG_ETRAX_AXISFLASHMAP is not configured, the flash rescue image
object file is empty. With recent versions of binutils, this results
in the following build error.

cris-linux-objcopy: error:
	the input file 'arch/cris/boot/rescue/rescue.o' has no sections

This is seen, for example, when trying to build cris:allnoconfig
with recently generated toolchains.

Since it does not make sense to build a flash rescue image if there is
no flash, only build it if CONFIG_ETRAX_AXISFLASHMAP is enabled.
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Fixes: 66ab3a74 ("CRIS: Merge machine dependent boot/compressed ..")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Jesper Nilsson <jesper.nilsson@axis.com>

arch/cris/boot/rescue/Makefile

@@ -10,6 +10,9 @@
 
 asflags-y += $(LINUXINCLUDE)
 ccflags-y += -O2 $(LINUXINCLUDE)
+
+ifdef CONFIG_ETRAX_AXISFLASHMAP
+
 arch-$(CONFIG_ETRAX_ARCH_V10) = v10
 arch-$(CONFIG_ETRAX_ARCH_V32) = v32
 
@@ -28,6 +31,11 @@ $(obj)/rescue.bin: $(obj)/rescue.o FORCE
 	$(call if_changed,objcopy)
 	cp -p $(obj)/rescue.bin $(objtree)
 
+else
+$(obj)/rescue.bin:
+
+endif
+
 $(obj)/testrescue.bin: $(obj)/testrescue.o
 	$(OBJCOPY) $(OBJCOPYFLAGS) $(obj)/testrescue.o tr.bin
 # Pad it to 784 bytes