Break install-types out of base

We support building elements without depending on the base element.
Breaking install-types out into its own element while making base depend
on it so elements can depend in it without base.

Change-Id: I104543d5482c76f60902e9fc32d91e196eeab51a

elements/base/element-deps

@@ -1,2 +1,3 @@
 dib-init-system
+install-types
 pkg-map

elements/install-types/README.rst

@@ -0,0 +1,49 @@
+=============
+install-types
+=============
+
+Enable install-types support for elements.
+
+Install types permit elements to be installed from different sources, such as
+git repositories, distribution packages, or pip. The default install type
+is 'source' but it can be modified on the disk-image-create command line
+via the --install-type option. For example you can set:
+
+    --install-type=package
+
+to enable package installs by default. Alternately, you can also
+set DIB\_DEFAULT\_INSTALLTYPE.
+
+Many elements expose different install types. The different implementations
+live under `<install-dir-prefix>-<install-type>-install` directories under an
+element's install.d. The base element enables the chosen install type by
+symlinking the correct hook scripts under install.d directly.
+`<install-dir-prefix>` can be a string of alphanumeric and '-' characters, but
+typically corresponds to the element name.
+
+For example, the nova element would provide:
+
+    nova/install.d/nova-package-install/74-nova
+    nova/install.d/nova-source-install/74-nova
+
+The following symlink would be created for the package install type:
+
+    install.d/74-nova -> nova-package-install/74-nova
+
+Or, for the source install type:
+
+    install.d/74-nova -> nova-source-install/74-nova
+
+All other scripts that exist under install.d for an element will be executed as
+normal. This allows common install code to live in a script under install.d.
+
+Environment Variables
+---------------------
+
+DIB_INSTALLTYPE_<install_dir_prefix>
+  :Required: No
+  :Default: source
+  :Description: Set the install type for the dir prefix. '-' characters can be
+    replaced with a '_'.
+  :Example: ``DIB_INSTALLTYPE_simple_init=repo`` Sets the simple-init element
+    install type to be repo.