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/pepper_defconfig b/configs/pepper_defconfig
index 44715be..a594929 100644
--- a/configs/pepper_defconfig
+++ b/configs/pepper_defconfig
@@ -3,6 +3,7 @@
 CONFIG_SPL_GPIO_SUPPORT=y
 CONFIG_SPL_EXT_SUPPORT=y
 CONFIG_SPL_FAT_SUPPORT=y
+CONFIG_SPL_I2C_SUPPORT=y
 CONFIG_SPL=y
 CONFIG_VERSION_VARIABLE=y
 CONFIG_HUSH_PARSER=y