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: Ic3c65a3e74e509133ddd99ee0c8702e410d225f5
Story: 2003447
Task:  24649
Depends-On: Ib39b8063e7759842ba15330c68503bfe2dea6e20
Signed-off-by: Scott Little <scott.little@windriver.com>

centos_iso_image.inc

@@ -0,0 +1,23 @@
+# List of packages to be included/installed in ISO
+# If these have dependencies, they will be pulled in automatically
+#
+
+# sm-common
+sm-common
+sm-common-libs
+sm-eru
+
+# sm-db
+sm-db
+
+# sm
+sm
+
+# sm-api
+sm-api
+
+# sm-client
+sm-client
+
+# sm-tools
+sm-tools