package/softether: New package

Package description was copied from
https://launchpad.net/~dajhorn/+archive/ubuntu/softether/



[Thomas:
 - reorganize patches by pulling the two biggest patches from Github,
   and only having the remaining patches in Buildroot.
 - use a full destination path when installing hamcorebuilder in
   $(HOST_DIR)/usr/bin.
 - minor reformatting.]

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>