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>

 # Overall toolchain configuration: paths, jobs, etc...

 # Allow unconditional usage of tristates

 config MODULES

     bool

     default y

 menu "Paths and misc options"

 source "config/global/ct-behave.in"

 source "config/global/paths.in"

 source "config/global/download.in"

 source "config/global/extract.in"

 source "config/global/build-behave.in"

 source "config/global/logging.in"

 endmenu