Fix unexpected script_path behaviour

When one calls the build.sh scripts with bash instead of the shebang $script_path contains the filepath, not the parent dir:

# cd /path
# grep -A2 script_path= build.sh
script_path=$(readlink -f "${0%/*}")
echo "$script_path"
exit 0
# ./build.sh
/path
# bash build.sh
/path/build.sh

This commit fixes that:

# grep -A2 script_path= build.sh
script_path="$( cd -P "$( dirname "$(readlink -f "$0")" )" && pwd )"
echo "$script_path"
exit 0
# ./build.sh
/path
# bash build.sh
/path