package/libefl: move to package directory

As discussed on the list move the last remaining package
in package/efl subdirectory to package/ directory.

- move the efl dependency to libefl.
- Set LIBEFL_VERSION with the version number directly.

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>