utils: allow using system wide toolchains cache

Out of the box, when building bootloaders, the scripts detect whether we have the needed toolchains and download them if needed, next to the source code.

This is great for development purposes because it only download the toolchains once.

However, on continous integration builds, where new sources are always downloaded, we also keep downloading the (non-changing) toolchains.

Introduce an environment variable SYSTEM_WIDE_TOOLCHAINS that points towards an existing, out-of-tree toolchain location.

This can be used to cache the toolchains instead of always re-downloading them.

Signed-off-by: Mattijs Korpershoek mkorpershoek@baylibre.com

On my setup (I have optical fiber), here is the overall build time diff: Without cache

build/build_all.sh --clean --mode=debug   171,15s user 43,60s system 207% cpu 1:43,71 total

With cache (using SYSTEM_WIDE_TOOLCHAINS)

build/build_all.sh --clean --mode=debug   159,34s user 40,63s system 269% cpu 1:14,23 total