Convert CONFIG_SCSI to Kconfig

This converts the following to Kconfig:
   CONFIG_SCSI

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/configs/Itead_Ibox_A20_defconfig b/configs/Itead_Ibox_A20_defconfig
index 4bae19f..bab25b4 100644
--- a/configs/Itead_Ibox_A20_defconfig
+++ b/configs/Itead_Ibox_A20_defconfig
@@ -15,6 +15,7 @@
 # CONFIG_SPL_DOS_PARTITION is not set
 # CONFIG_SPL_ISO_PARTITION is not set
 # CONFIG_SPL_EFI_PARTITION is not set
+CONFIG_SCSI=y
 CONFIG_ETH_DESIGNWARE=y
 CONFIG_SUN7I_GMAC=y
 CONFIG_USB_EHCI_HCD=y