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/k2e_evm_defconfig b/configs/k2e_evm_defconfig
index 9423b45..58c8c13 100644
--- a/configs/k2e_evm_defconfig
+++ b/configs/k2e_evm_defconfig
@@ -45,6 +45,7 @@
 CONFIG_SPL_POWER=y
 CONFIG_SPL_SPI_LOAD=y
 CONFIG_SYS_SPI_U_BOOT_OFFS=0x10000
+CONFIG_SPL_TARGET="u-boot-spi.gph"
 CONFIG_SYS_MAXARGS=64
 CONFIG_SYS_I2C_EEPROM_ADDR_LEN=2
 CONFIG_SYS_EEPROM_PAGE_WRITE_BITS=6