Convert CONFIG_SPL_LIBCOMMON_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/am335x_shc_prompt_defconfig b/configs/am335x_shc_prompt_defconfig
index 3bb83d0..8763bcc 100644
--- a/configs/am335x_shc_prompt_defconfig
+++ b/configs/am335x_shc_prompt_defconfig
@@ -1,6 +1,7 @@
 CONFIG_ARM=y
 CONFIG_TARGET_AM335X_SHC=y
 CONFIG_SPL_GPIO_SUPPORT=y
+CONFIG_SPL_LIBCOMMON_SUPPORT=y
 CONFIG_SPL_EXT_SUPPORT=y
 CONFIG_SPL_FAT_SUPPORT=y
 CONFIG_SPL_I2C_SUPPORT=y