exiv2: new package

Add support for Exiv2 library and utility to manage image metadata

Exiv2 can be built in GPLv2+ or commercial version. But in commercial
version the Nikon lens name database and the NLS support has to be
disabled for copyright reason.

[Thomas:
 - switch to using the CMake build system, which works better than the
   clunky autoconf + manual Makefile based build system.
 - add missing toolchains dependencies: does not build in static
   library configuration, needs C++, etc.]

Signed-off-by: Nicolas Serafini <nicolas.serafini@sensefly.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>