uboot-phytec

cmd: define CMD_DATA_SIZE when CONFIG_CMD_SETEXPR is selected

The setexpr shell command calls cmd_get_data_size() which is only built
when CMD_DATA_SIZE is defined. We need to define CMD_DATA_SIZE if
CONFIG_CMD_SETEXPR is selected or the build will fail if no other
command selecting this option is enabled.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>