exim: get rid of the convert the custom config file boolean knob

exim: get rid of the convert the custom config file boolean knob

The current implementation causes troubles with the autobuilders when they
set BR2_PACKAGE_EXIM_CUSTOM_CONFIG=y, because in these cases
BR2_PACKAGE_EXIM_CUSTOM_CONFIG_FILE is empty, and this is a configuration
error.

Solve the problem by entirely reomving the BR2_PACKAGE_EXIM_CUSTOM_CONFIG
boolean knob. Activation of the custon configuration file is now detected by
the fact that BR2_PACKAGE_EXIM_CUSTOM_CONFIG_FILE is non-empty.

Fixes:
  http://autobuild.buildroot.org/results/0db/0db54c555f6c7e635ebf99f86b3f91dc31e441f0/

[Thomas: slightly improve commit log.]

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