cmake: Fix "make VERBOSE=1"

It is desired behaviour that when 'make VERBOSE=1' is invoked, the
underlying scripts will be invoked in 'verbose mode', e.g. they have
the flag --verbose passed to them.

This patch modifies all the underlying scripts in the build system to
inject --verbose into the command line when the environment variable
VERBOSE is set.

The environment variable VERBOSE is a CMake concept. CMake will
generate Makefile's that read this environment variable and try to
behave accordingly. Unfortunately, the generated ninja build systems
behave differently and will have to be invoked like this:

VERBOSE=1 ninja -v

This fixes #4851

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>

scripts/gen_gdt.py

@@ -129,6 +129,8 @@ def parse_args():
     parser.add_argument("-o", "--output-gdt", required=True,
                         help="output GDT binary")
     args = parser.parse_args()
+    if "VERBOSE" in os.environ:
+        args.verbose = 1
 
 
 def main():