Convert CONFIG_SYS_SPI_U_BOOT_OFFS to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_SPI_U_BOOT_OFFS

Signed-off-by: Hannes Schmelzer <hannes.schmelzer@br-automation.com>
[trini: Expose this for SPL_SPI_SUNXI for now]
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/A20-OLinuXino-Lime2-eMMC_defconfig b/configs/A20-OLinuXino-Lime2-eMMC_defconfig
index 298c9f2..7e48328 100644
--- a/configs/A20-OLinuXino-Lime2-eMMC_defconfig
+++ b/configs/A20-OLinuXino-Lime2-eMMC_defconfig
@@ -15,6 +15,7 @@
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
 CONFIG_USE_PREBOOT=y
 CONFIG_SPL_I2C_SUPPORT=y
+CONFIG_SYS_SPI_U_BOOT_OFFS=0x8000
 CONFIG_CMD_DFU=y
 # CONFIG_CMD_FLASH is not set
 CONFIG_CMD_USB_MASS_STORAGE=y