samples: settings: Enable NVS backend on mimxrt1020_evk board

The tests for settings will not work properly with just provided config
file for mimxrt1020:
west build -p always -b mimxrt1020_evk samples/subsys/settings

as there the settings_save_one() would return -2 (-ENOENT) as by default
the CONFIG_SETTINGS_NONE is defined for mimxrt1020 configuration.

For proper operation (i.e. to have a memory medium to store settings
values) the NVS backend needs to be enabled.

Signed-off-by: Lukasz Majewski <lukma@denx.de>