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)

 --- glibc-2.7/sysdeps/unix/sysv/linux/kernel-features.h	2007-08-14 06:21:09.000000000 +0300

 +++ glibc-2.7/sysdeps/unix/sysv/linux/kernel-features.h.new	2008-07-23 07:05:20.000000000 +0300

 @@ -449,8 +449,7 @@

     and on Alpha just after 2.6.22-rc1.  */

  #if __LINUX_KERNEL_VERSION >= 0x020611 \

      && ((!defined __sh__ && !defined __alpha__) \

 -	|| (__LINUX_KERNEL_VERSION >= 0x020613 && defined __sh__) \

 -	|| (__LINUX_KERNEL_VERSION >= 0x020617 && defined __alpha__))

 +	|| (__LINUX_KERNEL_VERSION >= 0x020613 && defined __sh__))

  # define __ASSUME_ATFCTS	1

  #endif