mach-sc5xx: Update boot commands

mach-sc5xx: Update boot commands

Update the default boot commands to match the expected
bootargs in Linux and new SPI partitioning scheme.

Because the environment is no longer stored in the SPI
flash, imagesize has been removed and replaced with a fixed
length read to load from the SPI. Additionally the partitions
of the mmc have been updated. The first partition holds the fitImage
at /fitImage, and the second partition contains the rootfs.
With this change, the imagefile environment variable has also been
eliminated, the image in the first partition is expected to always
be named fitImage.

Signed-off-by: Caleb Ethridge <caleb.ethridge@analog.com>