package/jpeg-turbo: allow installing tools

package/jpeg-turbo: allow installing tools

Currently, the jpeg-turbo tools are forcibly removed, but they can be
usefull (e.g. to test performance on the target, or to script batch
conversions).

Add an option to allow keeping them.

Signed-off-by: Clément Péron <peron.clem@gmail.com>
[yann.morin.1998@free.fr
  - move the option out of the choice
  - move the option to a Config.in.options
  - drop the default 'n', which is ... the default
  - use positive logic to define the hook
  - move definition of the hook in the conditional block
  - don't indent the hook assignment
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>