build: rework verbosity level selection

V=99 and V=1 are now deprecated in favor of a new verbosity class system,
though the old flags are still supported.
You can set the V variable on the command line (or OPENWRT_VERBOSE in the
environment) to one or more of the following characters:

- s: stdout+stderr (equal to the old V=99)
- c: commands (for build systems that suppress commands by default, e.g. kbuild)
- w: warnings/errors only (equal to the old V=1)

SVN-Revision: 31484