toolchain: support gconv modules from glibc >= 2.34

toolchain: support gconv modules from glibc >= 2.34

Startig with glibc 2.34, the gconv modules description has been split in
two:
  - a common definition in the old location, /usr/lib/gconv/gconv-modules
  - specific definitions in a subdirectory, /usr/lib/gconv/gconv-modules.d/

This is done so as to simplify the handling of glibc gconv modules, and
eventually to segregate those outside of glibc, and so that third-parties
may also provide their own gconv converters and their definitions.

And starting with that same glibc version, most of the gconv modules
definitions are moved to an extra configuration file in that
sub-directory.

It is thus no longer possible to use special code pages, like cp850,
which are very useful to access FAT-formatted devices.

Add support for this new gconv layout, while keeping support for older
glibc versions. Note that the modules themselves are not moved or
renamed, just the definition files have changed.

Instead of passing the one old gonv modules definitions file on stdin,
we pass the base directory to that file, and move into the script the
responsibility to find all the gconv definition files.

Signed-off-by: Yann E. MORIN <[email protected]>
Cc: Thomas Petazzoni <[email protected]>
Cc: Romain Naour <[email protected]>
Cc: Thomas De Schampheleire <[email protected]>
Cc: Giulio Benetti <[email protected]>
Signed-off-by: Yann E. MORIN <[email protected]>