package/rauc: add a WantedBy section to the systemd unit file

package/rauc: add a WantedBy section to the systemd unit file

upstream-provided service has no WantedBy, which means that rauc won't
be enabled by default

Add a WantedBy instead of manually creating the symlink so the user can
enable/disable the unit properly

Signed-off-by: Jérémy Rosen <jeremy.rosen@smile.fr>
[yann.morin.1998@free.fr:
  - split long line
  - no need for continued line (no use for ';\' )
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>