toolchain: add option to copy the gconv libraries

The gconv libraries are used to translate between different character sets
('charsets', even 'csets' sometimes). Some packages need them to present
text to the user (eg. XBMC Gotham).

In (e)glibc they are implemented by the internal implemenation of iconv,
called gconv, and are provided as dlopen-able libraries.

Note that some gconv modules need extra libraries (shared by more than
one gconv module), so we must, when adding a subset of modules, scan the
installed modules in search of the missing libraries.

[Thomas: add general explanation in expunge-gconv-modules and fix
coding style.]

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Cc: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Cc: Eric Limpens <limpens@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>