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 -Narup uClibc-0.9.30.1.orig/libc/inet/getaddrinfo.c uClibc-0.9.30.1/libc/inet/getaddrinfo.c

 --- uClibc-0.9.30.1.orig/libc/inet/getaddrinfo.c	2009-02-26 21:49:14.000000000 +0900

 +++ uClibc-0.9.30.1/libc/inet/getaddrinfo.c	2009-03-06 00:59:23.000000000 +0900

 @@ -187,6 +187,8 @@ static unsigned __check_pf(void)

  		}

  		for (runp = ifa; runp != NULL; runp = runp->ifa_next) {

 +			if (runp->ifa_addr == NULL)

 +				continue;

  #if defined __UCLIBC_HAS_IPV4__

  			if (runp->ifa_addr->sa_family == PF_INET)

  				seen |= SEEN_IPV4;