build-binutils: build-gcc: disable shared

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
build-linux: separate headers installation and libc build

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
prepare: overlays: switch to consolidated overlays

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
split-overlay: produce consolidated overlays only

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>