package/oracle-mysql: fix path to systemd service file

The path is a remnant from before the rename of the package.

Rather than hard-code the path, use the available variable. Also use
that variable when copying the Sys-V init script.

Fixes:
    http://autobuild.buildroot.org/results/767/767f0be3894c45c1e6d18bc4528d0da7b50e3757/
    http://autobuild.buildroot.org/results/e93/e934683ebcb33c9cc8fe9de7b1d5dcd7866f3bce/
    http://autobuild.buildroot.org/results/5d0/5d04163de80f7900d4aed14cff75b2e0761f101d/



Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Ryan Coe <bluemrp9@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>