If a user has e.g. /tftpboot/x86 as directory and configures
base_dir=/tftboot and offset_dir=/x86, this leads to bad behavior
as the load module creates a symlink
/tftpboot/x86/<builddir> -> <absolut_builddir>
in this case instead of the desired
/tftpboot/x86 -> <absolut_builddir>
Furthermore, the module works on
/tftpboot/x86/config-00-00-00-00-00-00
and
/tftpboot/x86/<builddir>/config-00-00-00-00-00-00
afterwards, which looks bad too. As there is no warning at all, this can
be hard to debug. The commit adds an appropriate check with error message and
exit -1 on an existing directory.
Fixes#1630
