tools: Handle existing link to acpi directory

tools: Handle existing link to acpi directory

The link to acpi include directory is not removed by Makefile's 'clean'
target. This can lead to make failure when making xen/.dir target if
we try to create the link again.

We can prevent this failure by (1) removing acpi link when cleaning up
and (2) adding '-f' option to 'ln' (just like we do for other targets).

We should also add tools/include/acpi link to .gitignore.

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>