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)

Original patch from Gentoo.

-= BEGIN original header =-

-= END original header =-

diff -durN uClibc-0.9.30.orig/Rules.mak uClibc-0.9.30/Rules.mak

--- uClibc-0.9.30.orig/Rules.mak	2008-11-12 13:24:16.000000000 +0100

+++ uClibc-0.9.30/Rules.mak	2009-02-07 09:57:59.000000000 +0100

@@ -519,6 +519,7 @@

 endif

 CFLAGS += $(call check_gcc,-std=gnu99,)

+CFLAGS += $(call check_gcc,-fgnu89-inline,)

 LDFLAGS_NOSTRIP:=$(CPU_LDFLAGS-y) -Wl,-shared \

 	-Wl,--warn-common -Wl,--warn-once -Wl,-z,combreloc