package_rpm.bbclass: name postinst files with package name

package_rpm.bbclass: name postinst files with package name

When create rootfs, some post install scripts need be run on device.
They are saved under directory /etc/rpm-postinst and named with numbers
such as 100, 101 etc.

Update to name the postinst script file with its package name instead of
just number. That may be more easy to debug when there is a error.

[YOCTO #3218]

(From OE-Core rev: 9b2a008c25a7a3152ae2d8c64ae8de2534471d47)

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