system: make /bin/sh symlink use relative path

The symlink created by buildroot for /bin/sh is the full path to
a shell.  This does not match what busybox and other Linux
distributions do with using relative path symlinks. When handling
files on the host system this will point to the host system's shell
if present and not the target shell.

This is fixed by changing the /bin/sh symlink to use a relative path.

Fixes #8161.

[Thomas: minor fixups to the commit log suggested by Yann E. Morin.]

Signed-off-by: Matthew Starr <mstarr@hedonline.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>