tests: mesh_shell: Optimize prj.conf to fit in nrf51
tests: mesh_shell: Optimize prj.conf to fit in nrf51

Disable various LL features to be able to fit in RAM for the
nrf51_pca10028.

Signed-off-by: Carles Cufi <853081dc8dca4fe6a7e3c984752718b5db36e060@nordicsemi.no>

Vudentz/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,finikorg/zephyr,galak/zephyr,Vudentz/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,nashif/zephyr,finikorg/zephyr,Vudentz/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,nashif/zephyr,galak/zephyr,galak/zephyr,nashif/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr