when creating aliensrc packages, aliensrc_creator should follow symlinks

A source file in a bitbake recipe may be also a symlink. Currently, aliensrc_creator does not follow symlinks when creating the package, leading to errors (the actual file contents are not included in the package, while the checksum in aliensrc.json is calculated against the actual file)

example: rauc-1.5.1 recipe contains a symlink, and what is included in the aliensrc file is:

So instead of including the actual content of the ostc-insecure-cert.pem file, it includes a broken symlink

(aliens4friend add command "detected" this issue, beacause it found a checksum mismatch)