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/T1040D4RDB_SPIFLASH_defconfig b/configs/T1040D4RDB_SPIFLASH_defconfig
index e99809b..2a872b9 100644
--- a/configs/T1040D4RDB_SPIFLASH_defconfig
+++ b/configs/T1040D4RDB_SPIFLASH_defconfig
@@ -29,6 +29,7 @@
 CONFIG_CMD_DHCP=y
 CONFIG_CMD_MII=y
 CONFIG_CMD_PING=y
+CONFIG_CMD_ETHSW=y
 CONFIG_CMD_EXT2=y
 CONFIG_CMD_FAT=y
 CONFIG_FSL_CAAM=y