Makefile: Add Kconfig option CONFIG_SPL_IMAGE to select the SPL binary

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>