Package OpenSSH's sftp-server program, so we can install it as
dependency of dropbear in a future patch. Once sftp-server is installed,
the scp tool from OpenSSH can be used with the SFTP protocol, without
enabling the legacy flag for the legacy SCP protocol.
The sftp-server binary is 126K in size.
After packaging this, I realized that we could also have used
the openssh-sftp-server package from poky:
https://gitea.sysmocom.de/sysmo-bts/generic-poky/src/branch/pyro/meta/recipes-connectivity/openssh
But let's use this extra package now, it uses the most recent OpenSSH
source and doesn't build the rest of OpenSSH which we don't need.
Related: SYS#6403
Change-Id: I376bc31413935f7a560afd916a623228550dc6fe
Oliver Smith