package/chrony: use libedit for command line editing

chrony is licensed under GPL-2.0, which is not license compatible with
readline (GPL-3.0+), so remove the optional readline handling and replace
with libedit instead.

While we are at it, also explicitly disable the libedit backend when not
available.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>