rootfs-postcommands.bbclass: Only set DROPBEAR_RSAKEY_DIR once

rootfs-postcommands.bbclass: Only set DROPBEAR_RSAKEY_DIR once

If DROPBEAR_RSAKEY_DIR has already been set before, e.g. by overwriting
the file dropbear.default, the line will still be appended a second time.

DROPBEAR_RSAKEY_DIR="/path/to/dropbear"
DROPBEAR_EXTRA_ARGS="-B"
DROPBEAR_RSAKEY_DIR=/var/lib/dropbear

Signed-off-by: Michael Glembotzki <Michael.Glembotzki@iris-sensing.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>