package_rpm.bbclass: Set tmppath for rpm to somewhere which won't conflict with the...

package_rpm.bbclass: Set tmppath for rpm to somewhere which won't conflict with the rootfs

If the rootfs has a specific layout in mind for /var/tmp, it should be
free to create this. Preinst scripts trigger rpm to create its tmppath
directory which could potentially conflict with this.

We're seeing issues if the provider of the /var/tmp wants to use a symlink
and a preinst script executes before it has been installed. This change
places tmppath as used by rpm somewhere safe which is already automatically
cleaned up too.

(From OE-Core rev: 71e95c744eaa4dda1b3237db2e13f666f121c92b)

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