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."
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>
