[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.

src/user/srv.init/init.module

@@ -1,7 +1,7 @@
 # vim: ft=python
 
 init = module("srv.init",
-    targets = [ "user" ],
+    targets = [ "init" ],
     deps = [ "libc", "elf", "bootproto", "zstd" ],
     description = "Init server",
     ld_script = "init.ld",
@@ -14,4 +14,4 @@ init = module("srv.init",
         "pci.cpp",
         "service_locator.cpp",
         "start.s",
-    ])
+    ])