common_utils: create a common space to hold helper functions

Add a new script file to maintain all the helper functions at a common
place for better reachability and allowing other scripts to benefit
from it. Begin with moving symlink related helper functions from current
location at build-target-bins.sh to common_utils.sh

Signed-off-by: Sathyam Panda <sathyam.panda@arm.com>
Change-Id: Ie0789fe510c65817ec41937292a2738d317127b1