[build] Move init to its own target

In order to allow -fpic and -fpie in the user target, move init to it's
own target -- it needs its own special build rules to make it loadable
by boot.

assets/build/config.debug.yaml

@@ -0,0 +1,9 @@
+---
+ccflags: [
+  "-g3",
+  "-ggdb",
+]
+
+ldflags: [
+  "-g",
+]