docs/asciidoc: call $(pkgname) and $(pkgdir) in a single place

Like for all the package infrastructures, retrieve the package name and
directory in the front-end macro-variable, rather than everywhere in the
backend macro.

This allows us to clean up the ASCIIDOC macro, by removing all the calls
to $(pkgname) and $(pkgdir), and to UPPERCASE (which made the macro a
bit difficult to read.)

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Samuel Martin <s.martin49@gmail.com>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Reviewed-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>