vim script about concating a variable and a const string

I'm writing a function in my ~/.vimrc file, but I got a problem. I want to concat a variable and a const string, so I do this like below:

let linux_version=system('uname -r')
let host_kernel_dir= "/lib/modules/" . linux_version . "/build"
echo host_kernel_dir

I wanted result is /lib/modules/4.8.0-52-generic/build, but I got this result

"/lib/modules/4.8.0-52-generic

/build "

So it seems a \n was added. So how to get rid of this \n?