package/pkg-rebar: new infrastructure

Ease the development of packages that use the erlang rebar tool as
their build system.

Signed-off-by: Johan Oudinet <johan.oudinet@gmail.com>
[yann.morin.1998@free.fr: split the patch into semantically separated
patches; large rewrites of the rest]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

[Thomas, with help from Yann and Arnout:
 - Fix the comment about the symlink used to make sure rebar does not
   download dependencies. The comment was not up-to-date with where
   the symlink is actually created.
 - Make <pkg>_USE_BUNDLED_REBAR and <pkg>_USE_AUTOCONF be inherited by
   host packages from their corresponding target package.
 - Make sure host dependencies are inherited from the corresponding
   target packages dependencies. This requires copying some logic from
   inner-autotools-package and inner-generic-package, just like
   inner-autotools-package duplicates some logic from
   inner-generic-package.
 - Fix host variant of $(2)_BUILD_CMDS indentation, use double quotes
   instead of simple quotes. So that it matches the target
   $(2)_BUILD_CMDS, and what we do elsewhere in Buildroot.]

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>