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>