package/libegl: rename the _HAS and _PROVIDES variables

The basic rule for a package is to have its options named
after the package name. There is no reason this should not
also be the case for virtual packages.

Besides, this will allow us to switch libegl to use the
soon-to-be-introduced virtual-package infrastructure.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Cc: Mike Zick <minimod@morethan.org>
Cc: Samuel Martin <s.martin49@gmail.com>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
cc: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>