mk: fix verbosity zero

mk: fix verbosity zero

Verbosity is considered enabled when $V is not empty.
It is a well spread shortcut in makefiles, see git grep '$(if $(*V'
So V=0 and V=1 are equivalent.
It is fixed by unsetting V when it is 0.

A side effect is to fix kernel module compilation verbosity
which is set to 0 when V is empty.

Reported-by: Ferruh Yigit <ferruh.yigit@intel.com>
Signed-off-by: Thomas Monjalon <thomas.monjalon@6wind.com>
Acked-by: Ferruh Yigit <ferruh.yigit@intel.com>