grub-editenv: Add grub_util_readlink()

grub-editenv: Add grub_util_readlink()

Currently grub-editenv and related tools are not able to follow symbolic
links when finding their config file. For example the grub-editenv create
command will wrongly overwrite a symlink in /boot/grub2/grubenv with a new
regular file, instead of creating a file in the path the symlink points to.

A following patch will change that and add support in grub-editenv to
follow symbolic links when finding the grub environment variables file.

Add a grub_util_readlink() helper function that is just a wrapper around
the platform specific function to read the value of a symbolic link. This
helper function will be used by the following patch for grub-editenv.

The helper function is not added for Windows, since this operating system
doesn't have a primitive to read the contents of a symbolic link.

Signed-off-by: Peter Jones <pjones@redhat.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>