The sendmail Mail Filter API (Milter) is designed to allow third-party
programs access to mail messages as they are being processed in order to
filter meta-information and content.

This library is a prerequisite for the OpenDKIM package.

The proposed Makefile also contains sections for building Sendmail. These
sections are commented out because more work is necessary to properly
complete porting of Sendmail to LEDE/OpenWrt. The notes in Makefile
provide details on what is required to complete the port.

It has been verified that Sendmail executables compile and run properly
on the target system when the commented sections in Makefile are
uncommented.

Signed-off-by: Val Kulkov <val.kulkov@gmail.com>