build-sys: add a 'nonfs' phony to build a kernel image less than 3MB

The partition flash.os and flash.os2 of mtd flash are all 3MB.
If your guys need to deploy the kernel image into the flash you
need a kernel image less than 3MB. So we add the 'nonfs' phony
to disable the nfs and mount the /dev/sda4 as the rootfs.

In the sda4 condition you need to set the nvram varilable 'rootfs'
equal 'sata'

Signed-off-by: Jack Tan <jiankemeng@gmail.com>