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: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Tested-by: Breno Lima <breno.lima@nxp.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>