package/efl: rename libefl to efl

This allow to keep backward compatibility with older
defconfig files that used BR2_PACKAGE_EFL.

Don't add Config.in.legacy entry for libefl since it's
not part of any Buildroot release.

Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>