dhcp: fix bad --enable/disable-debug logic

It interprets disable as enable and wreaks havoc since it changes the
behaviour of the build, for instance not using configured leases files
paths.
Thanks to Nathaniel Roach for pointing me to this problem.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Reviewed-by: Nathaniel Roach <nroach44@gmail.com>
Tested-by: Nathaniel Roach <nroach44@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>