Add XML patching bitbake class.

This commit adds a new obmc-xmlpatch class that can be
inherited in a recipe to provide a do_patch task to patch XML
files.  The patch files are themselves XML, and must end in
.patch.xml for do_patch to find them.

This commit also includes Palmetto patches specified in
palmetto.xml.patch.xml that are required to build the system
inventory.

Change-Id: Idae6ffd8e7a3aa247115ff3a840e047727ff0d1a
Signed-off-by: Matt Spinler <spinler@us.ibm.com>
diff --git a/meta-palmetto/recipes-phosphor/mrw/mrw-native.bbappend b/meta-palmetto/recipes-phosphor/mrw/mrw-native.bbappend
new file mode 100644
index 0000000..836ef56
--- /dev/null
+++ b/meta-palmetto/recipes-phosphor/mrw/mrw-native.bbappend
@@ -0,0 +1,2 @@
+FILESEXTRAPATHS_prepend := "${THISDIR}/${PN}:"
+SRC_URI += "file://palmetto.xml.patch.xml"