efl: factorize version number as EFL_VERSION

All the EFL components are released simultaneously, with an identical
version number, just like all Qt5 components for example. So it makes
sense to have a single EFL_VERSION variable in package/efl/efl.mk that
is used by all the packages in package/efl/*/*.mk.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>