kbuild: do not print extra logs for V=2

Some scripts increase the verbose level when V=1, but others when
not V=0.

I think the former is correct because V=2 is not a log level but
a switch to print the reason for rebuilding.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>