machine: set constant UBI_VOLNAME value

Set UBI_VOLNAME to "rootfs" so that it is useful for using
a common kernel parameter.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
machine: remove the build of kernel as uImage: use zImage

zImage has many advantages over uImage and all bootloaders
can launch it: move to this default value for KERNEL_IMAGETYPE
which is why we remove its setup.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>