eal: cleanup lcore ID hand-over

eal: cleanup lcore ID hand-over

So far, a worker thread has been using its thread_id to discover which
lcore has been assigned to it.

On the other hand, as noted by Tyler, the pthread API does not strictly
guarantee that a new thread won't start running eal_thread_loop before
pthread_create writes to &lcore_config[xx].thread_id.

Though all OS implementations supported in DPDK (recently) ensure this
property, it is more robust to have the main thread directly pass
the worker thread lcore.

Signed-off-by: David Marchand <david.marchand@redhat.com>