evtest: disable documentation

evtest uses asciidoc to generate documentation, but /usr/bin/asciidoc
will try to use buildroot's host-python which doesn't have the required
unicodedata. To avoid that, define ASCIIDOC as an empty string so the
Makefile can't find the executable and skips the documentation
generation.  This is the same solution used in the evemu package.
Otherwise it will fail with this message:

make[1]: Entering directory `/br/output/build/evtest-1.32'
/usr/bin/make  all-am
make[2]: Entering directory `/br/output/build/evtest-1.32'
  File "/usr/bin/asciidoc", line 95
    except KeyError, k: return None

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>