exim: use a more standard build-time configuration

Buildroot currently ships a very minimal build configuration file for exim,
which disables most optional features. This is not coherent with the runtime
configuration file, taken verbatim from the exim distribution, which enables
some of these features.

The visible symptom is an error during boot that prevents exim from starting:

  Exim configuration error in line 541 of /etc/exim/configure:
    router dnslookup: cannot find router driver "dnslookup"

In order to fix this problem, we change the way exim is configured at build
time. Instead of blindly copying a minimal Buildroot-provided configuration
file, we now copy the exim-provided one and then tweak it to change the needed
options. This actually makes the configuration closer to standard exim.
As the amount of tweaking is remarkable, we also define a few macros to make
it easier and more readable.

This new approach was suggested by Bernd Kuhls.

Reported-By: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Cc: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>