Add OBMC_IMAGE_EXTRA_INSTALL variable

Prior to this patch packagegroup-core-boot was not being pulled in.
core-boot is what brings in the init_manager (systemd) but
somehow systemd was added as a dependency by OE for the skeleton package
so the missing core-boot packagegroup went unnoticed.

core-boot was not being pulled in because of an assignment to
IMAGE_INSTALL in the global configuration.  Yocto core provides a
mechanism to do the same thing safely - CORE_IMAGE_EXTRA_INSTALL;
use this and also adopt a similar approach for adding extra packages
to image recipes using obmc-phosphor-image.

Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/meta-phosphor/conf/machine/include/obmc-bsp-common.inc b/meta-phosphor/conf/machine/include/obmc-bsp-common.inc
index 260cc90..8d1fd7e 100644
--- a/meta-phosphor/conf/machine/include/obmc-bsp-common.inc
+++ b/meta-phosphor/conf/machine/include/obmc-bsp-common.inc
@@ -15,7 +15,7 @@
 
 IMAGE_FSTYPES += "overlay"
 IMAGE_FSTYPES += "cpio.${INITRAMFS_CTYPE}.u-boot"
-IMAGE_INSTALL += "u-boot-fw-utils"
+OBMC_IMAGE_EXTRA_INSTALL_append = " u-boot-fw-utils"
 
 IMAGE_CLASSES += "obmc-phosphor-image_types_uboot"