package_rpm.bbclass: add more description for pre/post scriptlets

package_rpm.bbclass: add more description for pre/post scriptlets

When write rpm spec file, one description line

"# pkgname - script_type"

is added to pre/post scriptlets for base package but no such line
writted for subpackage.

Add similiar line to subpackage to facilitate handling the pre/post
scriptlets.

Signed-off-by: Kang Kai <kai.kang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>