cmake: prefer CMAKE_BUILD_PARALLEL_LEVEL

cmake 3.12 introduced this environment variable. Prefer it to passing
PARALLEL_MAKE and PARALLEL_MAKEINST on the cmake command line, because
it gets passed to second stage cmake invocations while command-line
arguments do not (for example, multi-stage clang builds)

(From OE-Core rev: cdd44c93f02bb8cc2fa773e13c8ce36e3da23921)

Signed-off-by: Daniel McGregor <daniel.mcgregor@vecima.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>