Split image.inc across git repos

Currently compiling a new package and adding it
to the iso still requires a multi-git update because
image.inc is a single centralized file in the root git.

It would be better to allow a single git update to add
a package. Too allow this, image.inc must be split across
the git repos and the build tools must be changed to
read/merge those files to arrive at the final package list.

Current scheme is to name the image.inc files using this
schema.

${distro}_${build_target}_image_${build_type}.inc

distro = centos, ...
build_target = iso, guest ...
build_type = std, rt ...

Traditionally build_type=std is omitted from config files,
so we instread use ${distro}_${build_target}_image.inc.

Change-Id: I7c594c48ee74f8aceebe52f804b8670379acdf77
Story: 2003447
Task:  24649
Depends-On: Ib39b8063e7759842ba15330c68503bfe2dea6e20
Signed-off-by: Scott Little <scott.little@windriver.com>

centos_iso_image.inc

@@ -0,0 +1,36 @@
+# List of packages to be included/installed in ISO
+# If these have dependencies, they will be pulled in automatically
+#
+
+# compute-huge
+compute-huge
+
+# computeconfig
+computeconfig
+computeconfig-standalone
+computeconfig-subfunction
+
+# configutilities
+configutilities
+
+# controllerconfig
+controllerconfig
+
+# storageconfig
+storageconfig
+
+# cgts-client
+cgts-client
+
+# sysinv-agent
+sysinv-agent
+
+# sysinv
+sysinv
+
+# config-gate
+config-gate
+config-gate-compute
+
+# puppet-manifests
+puppet-manifests