Bluetooth: Mesh: Same config for all nrf51_qfaa boards in sample

Renames the nrf51_blenano config file to something generic and uses that
same config for the nrf51_ble400 board, which has the same constraints.

Cannot rely on the CONFIG_SOC_NRF51_QFAA variable, as the CONF_FILE
variable must be set before the boilerplate.

Signed-off-by: Trond Einar Snekvik <Trond.Einar.Snekvik@nordicsemi.no>