package_rpm.bbclass: fix and enhance the incremental rpm generation

package_rpm.bbclass: fix and enhance the incremental rpm generation

The incremental rpm generation usually broke when package_rpm.bbclass
changed, change its implementation to make it more stable:

* It depended on the previous and current saved manifest files in
  the past, it would break when the manifest changed. Now query the
  previous and current installed pkgs from rootfs/var/lib and
  rootfs/install/, this would be more reliable, the manifest's change
  would not affect it any more.

* Add explanations before package_install_internal_rpm to explain what
  does the function do.

* Remove an unwanted "awk '{print $1}'".

[YOCTO #2906]

(From OE-Core rev: 5d8ca498e09438bd91654fa8b8b2c970956d88e3)

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