ci: upgrade and refactor crosstool-ng builders The first commit upgrades our builders from crosstool-ng 1.24.0 to 1.25.0. There are otherwise no changes intended to the toolchains we're using, but there are some minor version upgrades as a result, especially GCC 8.3.0 to 8.5.0. The newer crosstool-ng will position us well to make toolchain upgrades in the future though, as we were maxed out before and it now goes up to GCC 11. The second commit refactors our config management to only commit a "mini-defconfig" for each target, produced by `ct-ng savedefconfig`. This makes it much clearer which settings we're actually changing, and also makes it easier to ensure consistency for things like mirror management.