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>

 # Bare metal config options

 ## select BARE_METAL

 ##

 ## help 'Bare metal' refers to those programs that run without any kernel.

 ## help

 ## help You probably want to say 'y' here if you plan to use your compiler

 ## help to build bootloaders. It is not yet suitable to build Linux kernels,

 ## help though, because the APCI stuff relies on the target C library headers

 ## help being available?!?!...