arm: mvebu: turris_mox: enable setexpr command in defconfig

Enable setexpr command in defconfig for Turris MOX. We have found that
this is sometimes useful in U-Boot scripts and there is enough space in
NOR memory on MOX.
Signed-off-by: NMarek Behún <marek.behun@nic.cz>
Reviewed-by: NStefan Roese <sr@denx.de>

configs/turris_mox_defconfig

@@ -32,7 +32,7 @@ CONFIG_CMD_PCI=y
 CONFIG_CMD_SPI=y
 CONFIG_CMD_USB=y
 CONFIG_CMD_WDT=y
-# CONFIG_CMD_SETEXPR is not set
+CONFIG_CMD_SETEXPR=y
 CONFIG_CMD_TFTPPUT=y
 CONFIG_CMD_CACHE=y
 CONFIG_CMD_TIME=y