lib/oe/utils: Return empty string in parallel_make

In cmake.bbclass we set CMAKE_BUILD_PARALLEL_LEVEL using parallel_make
function and if PARALLEL_MAKE is set to empty string then this variable
is exported as "None" causing cmake to fail with:
"'CMAKE_BUILD_PARALLEL_LEVEL' environment variable
invalid number 'None' given."

(From OE-Core rev: 56638a04899e35278c650da9929541ad7aafd4aa)

Signed-off-by: Tomasz Dziendzielski <tomasz.dziendzielski@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 2f790ded554a52ac18d1c28002142f9c62abec8b)
Signed-off-by: Steve Sakoman <steve@sakoman.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

1 files changed, 1 insertions, 1 deletions

diff --git a/meta/lib/oe/utils.py b/meta/lib/oe/utils.py
index 13f4271da0..83d298906b 100644
--- a/meta/lib/oe/utils.py
+++ b/meta/lib/oe/utils.py
@@ -193,7 +193,7 @@ def parallel_make(d, makeinst=False):
 
         return int(v)
 
-    return None
+    return ''
 
 def parallel_make_argument(d, fmt, limit=None, makeinst=False):
     """