Makefile: Don't export VERBOSE unless V=1 (for CMake)

CMake verbose mode is based on VERBOSE environment variable.
* If VERBOSE is exported but empty, only "Dependee ... is newer than
depender ..." messages are shown.
* If VERBOSE is exported and set (whatever the value), all compilation
commands are shown.

VERBOSE is currently systematically exported by Buildroot, even if it
is empty, in the root Makefile, which implies that the "light" verbose
mode - with "Dependee ... is newer than depender ..." messages - is
always enabled.

VERBOSE should only be exported when V=1, which is the standard way to
enable verbose mode in Buildroot.

Signed-off-by: Cédric Marie <cedric.marie@openmailbox.org>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>