libgit2: new package

libgit2: new package

Signed-off-by: Nicolas Cavallari <nicolas.cavallari@green-communications.fr>
[Thomas:
 - Do not select BR2_PACKAGE_ZLIB, because zlib is an optional
   dependency.
 - Handle optional dependencies in a more usual way in libgit2.mk:
   group the addition in _DEPENDENCIES and in _CONF_OPTS for a given
   library together.
 - libgit2 can optionally use libssh2, not libssh.
 - Add the optional dependency on zlib.
 - Always pass USE_ICONV=ON, the detection works perfectly fine, with
   both a C library providing iconv support built-in, and with
   libiconv. If neither provides iconv, it gets disabled automatically
   as expected.
 - Add libiconv as an optional dependency.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>