treewide: move build helper to more fitting places

Change-Id: I3d0cfe9283222d403ae369ec9db09201ad511e15
Reviewed-on: https://review.monogon.dev/c/monogon/+/3327
Reviewed-by: Serge Bazanski <serge@monogon.tech>
Tested-by: Jenkins CI
diff --git a/metropolis/node/core/update/update.go b/metropolis/node/core/update/update.go
index 28b2381..b6d2ce4 100644
--- a/metropolis/node/core/update/update.go
+++ b/metropolis/node/core/update/update.go
@@ -23,9 +23,9 @@
 	"google.golang.org/grpc/status"
 	"google.golang.org/protobuf/proto"
 
-	"source.monogon.dev/metropolis/node/build/mkimage/osimage"
 	abloaderpb "source.monogon.dev/metropolis/node/core/abloader/spec"
 	"source.monogon.dev/osbase/blockdev"
+	"source.monogon.dev/osbase/build/mkimage/osimage"
 	"source.monogon.dev/osbase/efivarfs"
 	"source.monogon.dev/osbase/gpt"
 	"source.monogon.dev/osbase/kexec"