omap2/3/4: Make omap3_defconfig usable on Nokia boards

The bootloader on Nokia boards does not support updating
the kernel cmdline. For the other boards, the bootloader
provided cmdline will be used instead.

To mount root on n8x0, compile in also the MMC_OMAP.
It seems that the best way to mount root with the
new kernels on Nokia boards is to mount root on mmcblk0p2.

Note that n8x0 current has several issues booting with
omap3_defconfig: The bootloader has a 2MB limit for loading
kernels, so you may want to use kexec. Also ARMv6 does
not currently work with ARMv7 for VFP and TLS.

Signed-off-by: Tony Lindgren <tony@atomide.com>