scripts: Fix encoding errors for python3

Moved call of decode('utf-8') as close as possible to
call of subprocess API to avoid calling it in a lot of
other places.

Decoded binary data to utf-8 where appropriate to fix devtool
and recipetool tests in python 3 environment.

(From OE-Core rev: 30d02e2aa2d42fdf76271234b2dc9f37bc46b250)

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/lib/devtool/__init__.py

@@ -69,6 +69,7 @@ def exec_watch(cmd, **options):
     buf = ''
     while True:
         out = process.stdout.read(1)
+        out = out.decode('utf-8')
         if out:
             sys.stdout.write(out)
             sys.stdout.flush()