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)

--- uClibc-0.9.30.orig/extra/scripts/install_headers.sh	2008-10-10 15:50:38.000000000 +0200

+++ uClibc-0.9.30/extra/scripts/install_headers.sh	2011-08-21 09:49:03.000000000 +0200

@@ -33,7 +33,7 @@

 (

 # We must cd, or else we'll prepend "$1" to filenames!

 cd "$1" || exit 1

-find ! -name '.' -a ! -path '*/.*' | sed -e 's/^\.\///' -e '/^config\//d' \

+find . ! -name '.' -a ! -path '*/.*' | sed -e 's/^\.\///' -e '/^config\//d' \

 	-e '/^config$/d'

 ) | \

 (