scripts: fix sysroot prefix dir

The sysroot prefix dir was broken in #4960f5d9f829 due to a mishap
when making the out-of-sysroot lib/ symlink: the './' was mistakenly
changed into a single '.' .

Although Jonathan suggested restoring the missing '/' to restore it to
normal operation, I prefered using an explicit pushd/popd to be extra
sure of the symlink location and target, along with a fix in the sysroot
relative directory calculation.

Reported-by: Jonathan Grundon <JGrundon@xos.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
(transplanted from e5fc5c9ea78af28d05244ba09cf718cf75470903)

 # This file adds functions to build the C library for bare metal (IE. none)

 # Copyright 2008 Yann E. MORIN

 # Licensed under the GPL v2. See COPYING in the root of this package

 do_libc_get() {

     :

 }

 do_libc_extract() {

     :

 }

 do_libc_check_config() {

     :

 }

 do_libc_start_files() {

     :

 }

 do_libc() {

     :

 }

 do_libc_finish() {

     :

 }