Convert CONFIG_SPL_TARGET to Kconfig

This converts the following to Kconfig:
   CONFIG_SPL_TARGET

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/P1010RDB-PB_SPIFLASH_defconfig b/configs/P1010RDB-PB_SPIFLASH_defconfig
index c32ccc2..a54f7ea 100644
--- a/configs/P1010RDB-PB_SPIFLASH_defconfig
+++ b/configs/P1010RDB-PB_SPIFLASH_defconfig
@@ -42,6 +42,7 @@
 CONFIG_SPL_ENV_SUPPORT=y
 CONFIG_SPL_I2C=y
 CONFIG_SPL_MPC8XXX_INIT_DDR=y
+CONFIG_SPL_TARGET="u-boot-with-spl.bin"
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PBSIZE=276
 CONFIG_CMD_IMLS=y