package/elementary: rename libelementary to elementary

The upstream name is elementary so use it.
Even if this package provide libelementary.so, it also provide
elementary_codegen, elementary_run and elm_prefs_cc tools.

Add a legacy entry.

Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>