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/uniphier_v7_defconfig b/configs/uniphier_v7_defconfig
index a2ea3a9..a448d1c 100644
--- a/configs/uniphier_v7_defconfig
+++ b/configs/uniphier_v7_defconfig
@@ -25,6 +25,7 @@
 CONFIG_SPL_STACK=0x100000
 CONFIG_SPL_NAND_SUPPORT=y
 CONFIG_SPL_NOR_SUPPORT=y
+CONFIG_SPL_TARGET="u-boot-with-spl.bin"
 CONFIG_CMD_CONFIG=y
 # CONFIG_CMD_XIMG is not set
 CONFIG_SYS_EEPROM_PAGE_WRITE_DELAY_MS=10