Recent versions of SDCC generate .lk files instead of .lnk - change the
OpenULINK Makefile "clean" target and top level .gitignore file to reflect
this.

Change-Id: I36f38638b712b962498c69c362f123378e1aa045
Signed-off-by: Martin Schmölzer <martin.schmoelzer@student.tuwien.ac.at>
Reviewed-on: http://openocd.zylin.com/1485


Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>