Incremental rpm image generation

Incremental rpm image generation, the rootfs would be totally removed and
re-created in the second generation by default, but with
INC_RPM_IMAGE_GEN = "1", the rpm based rootfs would be kept, and will do
update(remove/add some pkgs) on it.

NOTE: This is not suggested when you want to create a productive rootfs

For example:
  1) Add the follow config option to a conf file:
     INC_RPM_IMAGE_GEN = "1"

  2) bitbake core-image-sato
     modify a package
     bitbake core-image-sato

The rootfs would not be totally removed and re-created in the second
generation, it would be simply updated based on the "package".

Implatation:
1) Figure out the pkg which need to be removed or re-installed, then use
'rpm -e to remove the old one. Use the rpm's BUILDTIME to determine
which pkg has been rebuilt.

2) Figure out the pkg which is newly added, and use 'rpm -U' to install
it.

This only for the rpm based rootfs, the deb and ipk based rootfs would
be done later.

[YOCTO #1651]

(From OE-Core rev: 575ba3c9e153a1d8ac228a99a03ca2df5fbca151)

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/rootfs_rpm.bbclass

@@ -201,11 +201,15 @@ rootfs_check_package_exists() {
 }
 
 rootfs_install_packages() {
+    # The pkg to be installed here is not controlled by the
+    # package_install_internal_rpm, so it may have already been
+    # installed(e.g, installed in the first time when generate the
+    # rootfs), use '--replacepkgs' to always install them
 	for pkg in $@; do
 		${RPM} --root ${IMAGE_ROOTFS} -D "_dbpath ${rpmlibdir}" \
 			-D "__dbi_txn create nofsync private" \
 			--noscripts --notriggers --noparentdirs --nolinktos \
-			-Uhv $pkg || true
+			--replacepkgs -Uhv $pkg || true
 	done
 }