imx6 SMP enabled by default

[Ian Lepore]

As of r262695 option SMP is turned on in the IMX6 kernel config.  I also
added a new tunable so that if you have trouble that might be related to
SMP you can "set hw.ncpu=1" at the ubldr prompt or in /boot/loader.rc
and it will boot with a single core (you can set any number from 1 to
the number cores the chip has).

Also, FYI, you can put "set autoboot_delay=2" in /boot/loader.rc (or any
number you're happy with).  You can hit a key at any point while it's
loading the kernel and it will give you the interactive prompt before
booting, so a short delay works fine.

-- Ian