exim: allow using a custom configuration file

exim has lots of options configurable before the build process. It's not
useful to have all of them exposed in Buildroot, yet users may need to tweak
them.

Allow the user to pass an entire configuration file to make exim totally
reconfigurable without bloating the Buildroot menus.

[Thomas: remove "for exim" from Config.in options prompts.]

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>