Convert CONFIG_CMD_ETHSW to Kconfig

This converts the following to Kconfig:
   CONFIG_CMD_ETHSW

Also enable it for sandbox to increase build coverage.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/T1040QDS_DDR4_defconfig b/configs/T1040QDS_DDR4_defconfig
index 6ebde4d..063ae20 100644
--- a/configs/T1040QDS_DDR4_defconfig
+++ b/configs/T1040QDS_DDR4_defconfig
@@ -20,6 +20,7 @@
 CONFIG_CMD_DHCP=y
 CONFIG_CMD_MII=y
 CONFIG_CMD_PING=y
+CONFIG_CMD_ETHSW=y
 CONFIG_CMD_BMP=y
 CONFIG_CMD_DATE=y
 CONFIG_CMD_EXT2=y