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)

 diff -durN glibc-2.12.1.orig/elf/dl-lookup.c glibc-2.12.1/elf/dl-lookup.c

 --- glibc-2.12.1.orig/elf/dl-lookup.c	2009-03-30 23:14:32.000000000 +0200

 +++ glibc-2.12.1/elf/dl-lookup.c	2009-11-13 00:51:36.000000000 +0100

 @@ -301,6 +301,12 @@

  	      /* FALLTHROUGH */

  	    case STB_GLOBAL:

 	    success:

 +#ifdef __mips__

 +	      /* HACK: MIPS marks its lazy evaluation stubs with SHN_UNDEF

 +		 symbols, we skip them. */

 +	      if (sym->st_shndx == SHN_UNDEF)

 +		break;

 +#endif

  	      /* Global definition.  Just what we need.  */

  	      result->s = sym;

  	      result->m = (struct link_map *) map;