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 -dur strace-4.5.16.orig/net.c strace-4.5.16/net.c

 --- strace-4.5.16.orig/net.c	2006-12-13 20:57:23.000000000 +0100

 +++ strace-4.5.16/net.c	2007-07-14 19:29:36.000000000 +0200

 @@ -800,8 +800,12 @@

  #ifdef SOL_PACKET

  static const struct xlat sockpacketoptions[] = {

 +#if defined(PACKET_ADD_MEMBERSHIP)

  	{ PACKET_ADD_MEMBERSHIP,	"PACKET_ADD_MEMBERSHIP"	},

 +#endif

 +#if defined(PACKET_DROP_MEMBERSHIP)

  	{ PACKET_DROP_MEMBERSHIP,	"PACKET_DROP_MEMBERSHIP"},

 +#endif

  #if defined(PACKET_RECV_OUTPUT)

  	{ PACKET_RECV_OUTPUT,		"PACKET_RECV_OUTPUT"	},

  #endif