Support pluggable output sinks for Printf with a ring-buffer fallback

The implementation of Printf has been moved from the early package to
the kfmt package. The dependency to ActiveTerminal has been removed and
the code now uses an io.Writer for its output. As Go interfaces cannot
be used before bootstrapping the Go runtime, the code uses a ring-buffer
fallback for storing any kernel output emitted before that point.

Makefile

@@ -170,6 +170,7 @@ lint: lint-check-deps
 		--deadline 300s \
 		--exclude 'return value not checked' \
 		--exclude 'possible misuse of unsafe.Pointer' \
+		--exclude 'x \^ 0 always equals x' \
 		./...
 
 lint-check-deps: