evtest: don't build man page

evtest: don't build man page

Just selecting this package resulted in a build error:

/home/mhei/openwrt.git/staging_dir/hostpkg/bin/python3: No module named asciidoc
make[4]: *** [Makefile:856: evtest.xml] Error 1

Since we usually do not need the man page, just prevent to
build it by pre-setting two environments variables. Then
the makefile warns about, but don't try to build.

Signed-off-by: Michael Heimpold <mhei@heimpold.de>