ast2600: Configure u-boot-with-spl.bin target

The normal way of loading u-boot is as a FIT, so configure u-boot.img as
the SPL playload.

The u-boot-with-spl.bin target will add padding according to
CONFIG_SPL_MAX_SIZE which defaults to 64KB on the AST2600.

With this the following simple steps can be used to build and boot a
system:

  make u-boot-with-spl.bin
  truncate -s 64M u-boot-with-spl.bin
  qemu-system-arm -nographic -M ast2600-evb \
    -drive file=u-boot-with-spl.bin,if=mtd,format=raw

Reviewed-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Chia-Wei Wang <chiawei_wang@aspeedtech.com>
Signed-off-by: Joel Stanley <joel@jms.id.au>
diff --git a/configs/evb-ast2600_defconfig b/configs/evb-ast2600_defconfig
index 588d78e..970d356 100644
--- a/configs/evb-ast2600_defconfig
+++ b/configs/evb-ast2600_defconfig
@@ -21,6 +21,8 @@
 CONFIG_SPL=y
 # CONFIG_ARMV7_NONSEC is not set
 CONFIG_SYS_LOAD_ADDR=0x83000000
+CONFIG_SPL_PAYLOAD="u-boot.img"
+CONFIG_BUILD_TARGET="u-boot-with-spl.bin"
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
 CONFIG_FIT=y
 CONFIG_SPL_FIT_SIGNATURE=y