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