openssh: add variable for key path

openssh: add variable for key path

This patch adds a variable for the key directory path. This is especially
useful when working with a read-only file system where you want to
specify the location e.g. on a r/w partition. To be consistent, the
change was also done for the read write path.

For changing the path simply create a bbappend and override the
variable.

Signed-off-by: Patrick Vogelaar <patrick.vogelaar.dev@mailbox.org>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>