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 instead use ${distro}_${build_target}_image.inc.

Change-Id: I935a0abfea7f3799becd18d172d412e09eb4d55c
Story: 2003447
Task:  24649
Depends-On: Ib39b8063e7759842ba15330c68503bfe2dea6e20
Signed-off-by: Scott Little <scott.little@windriver.com>

centos_guest_image.inc

@@ -0,0 +1,16 @@
+# List of packages to be included/installed in guest image
+# If these have dependencies, they will be pulled in automatically
+#
+
+#
+# guest extensions
+#
+
+# guest-client
+guest-client
+
+# guest-scale-agent
+guest-scale-agent
+
+# host-guest-comm
+guest-host-comm

centos_guest_image_rt.inc

@@ -0,0 +1,16 @@
+# List of packages to be included/installed in RT guest image
+# If these have dependencies, they will be pulled in automatically
+#
+
+#
+# guest extensions
+#
+
+# guest-client
+guest-client
+
+# guest-scale-agent
+guest-scale-agent
+
+# host-guest-comm
+guest-host-comm

centos_iso_image.inc

@@ -0,0 +1,20 @@
+# List of packages to be included/installed in ISO
+# If these have dependencies, they will be pulled in automatically
+#
+
+# nova-api-proxy
+nova-api-proxy
+
+# guest-scale-agent
+guest-scale-helper
+
+# host-guest-comm
+host-guest-comm
+
+# nfv
+nfv
+nfv-common
+nfv-plugins
+nfv-tools
+nfv-vim
+nfv-client