linux/perf: really do not build the documentation

linux/perf: really do not build the documentation

The perf buildsystem, inside the kernel, is not really amenable to be
easily used...

Regarding the documentation, it will forcefully try to detect asciidoc
and, with the latest versions, xmlto, completely disregarding what the
user may provide.

We currently pass ASCIIDOC= (the empty string) on the make command line,
as an attempt to disable building the documentation, but that has no
effect whatsoever on perf: that variable is not passed down to the
sub-sub-make (yes, a two-level depth) that is responsible for building
the documentation.

We really do not want to build any of the documentation (the user can
refer to the documentation on his own development machine), so we use a
little dirty trick: we provide a GNUmakefile beside the existing
Makefile for the documentation; GNUmakefile always takes precedence over
a Makefile when both are present. We only provide a catch-all-no-recipe
rule in that GNUmakefile, so it really does nothing useful, except avoid
building the documentation.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>