Switch wandboards over to the common IMX6 kernel config, which has SMP
enabled.  Also switch IMX6 to use SCHED_ULE.

The now-unreferenced WANDBOARD.common config will be deleted after giving
folks who may be including it a heads-up to switch to IMX6.
Enable SMP for Pandaboard.
Initialise m_pkthdr via bzero instead of explicitly zeroing each member

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