core: move reproducible option to advanced sub-menu

The reproducible build is only in its infancy; it is far from being
complete.

As such, move it to the "advanced" build sub-menu, where it belong for
now...

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