build-sys: implement the auto-detect the numbers of cpu

Implement the auto-detect the numbers of cpu and parallelize
the build.

Signed-off-by: Jack Tan <jiankemeng@gmail.com>
build-sys: update the makefile to enable menuconfig

add the feature of checking .config before build the kernel

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

add the new intructions of setup nfs and sata rootfs

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