package/exiv2: cleanup options and licenses

package/exiv2: cleanup options and licenses

exiv2 no longer requires a non commercial option for lens database
integration since version 0.27. See [1] and [2]

The BR2_PACKAGE_EXIV2_LENSDATA option is maintained because the
src/nikonmn_int.cpp file always specifies that the Nikon lens name
database is free to use in non-commercial, GPL or open source software
only.

Legacy handling for the removed option COMMERCIAL is not needed, since
now it's always enabled.

Add the dedicated BSD-3-Clause license file for CMakeLists.txt,
config/FindEXPAT.cmake and config/FindMSGFMT.cmake files.

[1] https://github.com/Exiv2/exiv2/commit/07f63003b7f50dd7d8719585ba54623e0f60c3c4
[2] https://github.com/Exiv2/exiv2/commit/085d8a309aafb8ed5a72cad027a1a0ea974938d9

Signed-off-by: Nicolas Serafini <nicolas.serafini@sensefly.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>