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/Kconfig b/Kconfig
index 305b265..7a5491b 100644
--- a/Kconfig
+++ b/Kconfig
@@ -224,6 +224,16 @@
 	  which are not shipped in the U-Boot source tree.
 	  Please, see doc/README.x86 for details.
 
+config SPL_IMAGE
+	string "SPL image used in the combined SPL+U-Boot image"
+	default "spl/boot.bin" if ARCH_AT91 && SPL_NAND_SUPPORT
+	default "spl/u-boot-spl.bin"
+	help
+	  Select the SPL build target that shall be generated by the SPL
+	  build process (default spl/u-boot-spl.bin). This image will be
+	  used to generate a combined image with SPL and main U-Boot
+	  proper as one single image.
+
 config BUILD_TARGET
 	string "Build target special images"
 	default "u-boot-with-spl.sfp" if TARGET_SOCFPGA_ARRIA10
@@ -232,6 +242,7 @@
 	default "u-boot-elf.srec" if RCAR_GEN3
 	default "u-boot.itb" if SPL_LOAD_FIT && ARCH_SUNXI
 	default "u-boot.kwb" if KIRKWOOD
+	default "u-boot-with-spl.bin" if ARCH_AT91 && SPL_NAND_SUPPORT
 	help
 	  Some SoCs need special image types (e.g. U-Boot binary
 	  with a special header) as build targets. By defining