tools/automake: verbose variable compatibility

tools/automake: verbose variable compatibility

An old patch attempted to harmonize the way that
both Openwrt and Automake uses the $(V) variable.
However, it was reverted because of the side-effects.

This method is more simple and just
allows Automake to accept any string
as part of the verbosity toggle,
falling back to the default if null.

Ref: e6901bf90 ("tools/automake: Revert "Do not use $(V) - force AM_V=1"")
Ref: 43365ca66 ("Do not use $(V) - force AM_V=1")
Signed-off-by: Michael Pratt <mcpratt@pm.me>