toolchain: always switch to latest GCC version

Normally Gentoo expects moving between major GCC releases to be a manual
step. In our case we want this to always be automatic, otherwise the GCC
version won't be switched at all.
prod_image_util: update /etc/ld.so.conf on every boot

Early images created this as a file instead of a symlink, these systems
will not be able to find the new GCC libraries using the older config.
Merge pull request #322 from marineam/gcc

GCC update fixes