package_rpm: Only rebuild the indexes if the packages have changed

package_rpm: Only rebuild the indexes if the packages have changed

This change farms the solvedb creation out to a separate script which
handles creation of the index, only if mtime of any of the packages
has changed.

For a core-image-minimal set of rpm's this saves ~20s of a 45s rootfs
build. For core-image-sato it saves 1 minute of a 5 minute rootfs build.
The more packages in the system, the bigger the saving will be.

(From OE-Core rev: 3021136e7b42ab64ca16f30c88467c4b00d51ee0)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>