Convert CONFIG_SPL_I2C_SUPPORT to Kconfig

Move this option to Kconfig and tidy up existing uses.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/Itead_Ibox_A20_defconfig b/configs/Itead_Ibox_A20_defconfig
index 9adebf5..1a8fad7 100644
--- a/configs/Itead_Ibox_A20_defconfig
+++ b/configs/Itead_Ibox_A20_defconfig
@@ -1,5 +1,6 @@
 CONFIG_ARM=y
 CONFIG_ARCH_SUNXI=y
+CONFIG_SPL_I2C_SUPPORT=y
 CONFIG_MACH_SUN7I=y
 CONFIG_DRAM_CLK=480
 CONFIG_MMC0_CD_PIN="PH1"