Check that /srv/tftpboot/suse-11.3/install is not a symlink

Using a symlink is not supported for PXE boot, unfortunately.

https://bugzilla.novell.com/show_bug.cgi?id=886196
Merge pull request #2049 from vuntz/no-symlink

Check that /srv/tftpboot/suse-11.3/install is not a symlink
Fix type of crowbar-applied attribute that we add on install
Merge pull request #2050 from vuntz/crowbar-applied

Fix type of crowbar-applied attribute that we add on install
fix doc links in README.md
Merge pull request #2051 from fly/master

fix doc links in README.md
network-json-validator: Validate interface names too
Merge pull request #2052 from vuntz/validate-ifaces

network-json-validator: Validate interface names too
sledgehammer: Fix regexp to detect boot entries

It's some hexadecimal number we're looking for.
sledgehammer: Make sure that MAC variable is empty unless set

Some code path was not doing that, which could result in using a
pre-existing env var.
Merge pull request #2053 from vuntz/sledgehammer-fixes

sledgehammer: Fix regexp to detect boot entries
Run zypper commands in non-interactive mode

Otherwise it might be stalling forever on input that
is never going to come.