remove busybox

remove busybox , because it's no need.

Signed-off-by: Lindell Xu <lxu@vetronicsltd.com.cn>
add shell for loop usages

for loop usages:
	double brackets
	in seq
	while
etc.

Signed-off-by: Lindell Xu <lxu@vetronicsltd.com.cn>