Prep for Buildroot 2020.08 by locking GCC to v9
Prep for Buildroot 2020.08 by locking GCC to v9

Buildroot 2020.08 changes the GCC default to 10, so locking it to 9 in
the `nerves_defconfig` makes the system build under both the current
version of Buildroot, 2020.05.1, and the upcoming version.

nerves-project/nerves_system_rpi0,nerves-project/nerves_system_rpi0