getting_started: changes for custom toolchains
getting_started: changes for custom toolchains

This option uses a lot of typing, so point out there's a way to save
keystrokes using -C.

Tweak the language around the target toolchain.

Signed-off-by: Marti Bolivar <9d90ac4c7bf6a305f6bfd81a23c7859bc883380e@nordicsemi.no>

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