[Buildroot] Using a bootlin toolchain with buildroot

[Jan Kundrát]

Here's what works well for us:

BR2_TOOLCHAIN_EXTERNAL=y
BR2_TOOLCHAIN_EXTERNAL_DOWNLOAD=y
BR2_TOOLCHAIN_EXTERNAL_URL="https://toolchains.bootlin.com/downloads/releases/toolchains/x86-i686/tarballs/x86-i686--glibc--bleeding-edge-2018.02-1.tar.bz2"
BR2_TOOLCHAIN_EXTERNAL_CUSTOM_PREFIX="i686-buildroot-linux-gnu"
BR2_TOOLCHAIN_EXTERNAL_GCC_7=y
BR2_TOOLCHAIN_EXTERNAL_HEADERS_4_9=y
BR2_TOOLCHAIN_EXTERNAL_CUSTOM_GLIBC=y
BR2_TOOLCHAIN_EXTERNAL_CXX=y

Cheers,
Jan