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/stm32mp15_dhcom_basic_defconfig b/configs/stm32mp15_dhcom_basic_defconfig
index 38f1643..72d2f6e 100644
--- a/configs/stm32mp15_dhcom_basic_defconfig
+++ b/configs/stm32mp15_dhcom_basic_defconfig
@@ -49,6 +49,7 @@
 CONFIG_SYS_SPI_U_BOOT_OFFS=0x80000
 CONFIG_SPL_USB_GADGET=y
 CONFIG_SPL_DFU=y
+CONFIG_SPL_TARGET="u-boot.itb"
 CONFIG_SYS_PROMPT="STM32MP> "
 CONFIG_SYS_PBSIZE=1050
 # CONFIG_CMD_ELF is not set