The OpenDKIM package provides a service for signing and verifying
DomainKeys Identified Mail (DKIM) signatures. OpenDKIM consists of
a library that implements the DKIM service and a milter-based
filter application that can plug in to any milter-aware MTA, such
as Postfix or Sendmail, to provide that service to sufficiently
recent sendmail MTAs and other MTAs that support the milter
protocol.

This submission provides three new packages:
- libopendkim, a library for signing and verifying DKIM signatures,
- opendkim, the server application and the genkey script,
- opendkim-tools, a set of tools for configuring and testing OpenDKIM.

While at it, add PKG_BUILD_DEPENDS statement to sendmail's Makefile.
Travis CI buildbot sometimes fails to compile libopenssl before
starting to build sendmail. Since sendmail depends on libopenssl, the
whole Travis CI build process fails. Setting PKG_BUILD_DEPENDS
to "openssl", the directory name of libopenssl's Makefile, fixes the
problem.

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

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>