package/parted: relax dependency on readline

parted can be configured without support for readline (ie. can be
not interactive), so we can relax the dependency on readline, and
make it an optional feature.

(Based on a code-snippet from Thomas.)

Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Jerzy Grzegorek <jerzy.grzegorek@trzebnica.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>