update-all.sh, install.sh: rename into sync{,-installed}.sh and reuse code.
Makefile.main: reduce logging verbosity and redundancy.
Ditch $BUILDROOT.

Other top-level directories are hardcoded; why keep complexity for the
last one?
Remove prefetch-*.sh (unneeded).

utils/prefetch-pkg.sh is left as is.
Makefile.template-{aur,pacman}: add possibility to skip build/install steps.

Also improve error handling in Makefile.template-pacman.
utils/generate-makefile.sh: rename FORCE_REBUILD -> BUILD_FORCE for consistency.
Add package export and foreign build support.

- allow skipping build/install stages:
BUILD_MODE=fetch|build|install for sync.sh
- allow export-building for different -march (skips install, forces
rebuild and places package into a special directory):
BUILD_EXPORT=<arch> for sync.sh