dhcp: disable isc assertions (reproducible builds)

The isc assertions from the bundled bind dns library are
using the __FILE__ macro for debug messages (see
dhcp-4.3.5/bind/bind-9.9.9-P3/lib/isc/include/isc/assertions.h).

Disabling the assertions gains:

- reproducible builds (no build time paths in the executable)
- space saving on the target:
  dhcpd: 1.9M -> 1.6M
  dhcrelay: 1.6M -> 1.3M

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>