warp7: defconfig: Enable CMD_SETEXPR

setexpr allows us to do arithmetic for env variables - something that is
both useful and required when doing HAB authentication without hard-coding
HAB load addresses.

This patch enables CMD_SETEXPR for the WaRP7 defconfig.
Signed-off-by: NBryan O'Donoghue <bryan.odonoghue@linaro.org>
Tested-by: NBreno Lima <breno.lima@nxp.com>
Reviewed-by: NFabio Estevam <fabio.estevam@nxp.com>

configs/warp7_defconfig

@@ -20,7 +20,7 @@ CONFIG_CMD_MMC=y
 CONFIG_CMD_PART=y
 CONFIG_CMD_USB=y
 CONFIG_CMD_USB_MASS_STORAGE=y
-# CONFIG_CMD_SETEXPR is not set
+CONFIG_CMD_SETEXPR=y
 CONFIG_CMD_DHCP=y
 CONFIG_CMD_CACHE=y
 CONFIG_CMD_EXT2=y