Makefile: Add Kconfig option CONFIG_SPL_IMAGE to select the SPL binary

This patch adds the CONFIG_SPL_IMAGE option to select the SPL image that
shall be used to generate the combined SPL + U-Boot image. The default
value is the current value "spl/u-boot-spl.bin".

This patch also sets CONFIG_SPL_IMAGE to "spl/boot.bin" for AT91 targets
which use SPL NAND support (boot from NAND). For these build targets the
combined image "u-boot-with-spl.bin" is now automatically generated and
can be programmed into NAND as one single image (vs. SPL image and U-Boot
as 2 separate images).

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Heiko Schocher <hs@denx.de>
Cc: Andreas Bießmann <andreas@biessmann.org>
Cc: Eugen Hristev <eugen.hristev@microchip.com>
diff --git a/Makefile b/Makefile
index aedc2ea..8f59c12 100644
--- a/Makefile
+++ b/Makefile
@@ -1232,9 +1232,11 @@
 SPL_PAYLOAD := u-boot.bin
 endif
 
+SPL_IMAGE := $(CONFIG_SPL_IMAGE:"%"=%)
+
 OBJCOPYFLAGS_u-boot-with-spl.bin = -I binary -O binary \
 				   --pad-to=$(CONFIG_SPL_PAD_TO)
-u-boot-with-spl.bin: spl/u-boot-spl.bin $(SPL_PAYLOAD) FORCE
+u-boot-with-spl.bin: $(SPL_IMAGE) $(SPL_PAYLOAD) FORCE
 	$(call if_changed,pad_cat)
 
 ifeq ($(CONFIG_ARCH_LPC32XX)$(CONFIG_SPL),yy)