package/openblas: allow disabling multithreading

Buildroot would automatically enable multithreading in OpenBLAS if the
architecture supports it. However, one may want to avoid OpenBLAS creating
threads itself and configure single-threaded operation. To accommodate this
use case, add a config option for multithreading.

When multithreading is disabled but OpenBLAS functions are called in the
same application by multiple threads, then locking is mandatory. The
USE_LOCKING flag was added in version 0.3.7 with following release note:

    a new option USE_LOCKING was added to ensure thread safety when OpenBLAS
    itself is built without multithreading but will be called from multiple
    threads.

However, if one knows that OpenBLAS will only be called from single-threaded
applications, then passing USE_LOCKING is not necessary, so make it a config
option too.

When multithreading is enabled, locking is implicitly enabled inside
openblas, so only provide the locking option when multithreading is
disabled.

Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>