package_rpm.bbclass: fix incremental rpm image generation

package_rpm.bbclass: fix incremental rpm image generation

* Check ${target_rootfs}/etc/passwd rather than
  ${target_rootfs}${rpmlibdir} to make sure that it has been previously
  installed.

* Remove the "--nodeps" when incremental image generation, it should
  take care of the dependencies. Still use "--replacefiles --replacepkgs" in
  case there are conflicts.

[YOCTO #3047]

(From OE-Core rev: 2b3df2ec7979a49842df172be442a8794fe68fff)

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