flannel: new package

Flannel is a virtual network that gives a subnet to each host for use
with container runtimes.

Cc: Brandon Philips <brandon.philips@coreos.com>
Signed-off-by: Geoff Levand <geoff@infradead.org>
[Thomas:
  - Don't select BR2_PACKAGE_HOST_GO since this option was removed.
  - Take into account the rename of BR2_PACKAGE_GO_ARCH_SUPPORTS to
    BR2_PACKAGE_HOST_GO_ARCH_SUPPORTS.
  - Rewrap Config.in help text, it was slightly too large.
  - Don't use the github function (the link was anyway incorrect), but
    instead a direct link to the tarball, since the tarballs are
    uploaded by the maintainers.
  - Update the hash accordingly.
  - Use $() to reference make variables and not ${}.
  - Call the go compiler from $(HOST_DIR)/usr/bin, rather than from
    HOST_GO_ROOT.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>