package/pkg-rebar.mk: remove dependencies from rebar.config

package/pkg-rebar.mk: remove dependencies from rebar.config

Instead of having a patch in every rebar package to remove the
dependencies in the rebar.config file in order to avoid rebar
downloading such dependencies at build time, implement it directly
as a post-patch hook in the rebar infrastructure.
Add a way to explicitly deactivate this behavior if any package needs
such lines in the rebar.config file.

Signed-off-by: Johan Oudinet <johan.oudinet@gmail.com>
[Thomas:
 - rename macro to remove-rebar-config-dependencies
 - move the macro outside the inner-rebar-package, so that it is
   declared with the other utility macros found in pkg-rebar.mk]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>