mvebu: venom resize kernel to 6MB

mvebu: venom resize kernel to 6MB

venom has a 3MB kernel partition as specified by the DTS.
3MB is not sufficient for building with many kernel modules or newer
kernel versions.

venom uboot however as set from factory will load up to 6MB.
This can be observed by looking a uboot log:
	NAND read: device 0 offset 0x900000, size 0x600000
	6291456 bytes read: OK
and from uboot environment variables:
	$ fw_printenv | grep "priKernSize";
	priKernSize=0x0600000

Resize the root partitions from 120MB to 117MB to let kernel expand
into it another 3MB.
And set kernel target size to 6MB.

Lastly set the kernel-size-migration compatibility version on venom to
prevent sysupgrading without first reinstalling from a factory image.

Signed-off-by: Tad Davanzo <tad@spotco.us>