components/primary_compute,safety_island: Fix packet loss from Linux to Zephyr

On the Linux sender, the rpmsg_net_start_xmit function in the
rpmsg_net module needs to return an error code to notify the upper
layer when a transmission fails. This packet will be left in the
network queue for the next scheduling.

On the Zephyr receiver, when the network buffer is exhausted, the
RPMsg processing thread will be blocked until the network buffer
can be successfully allocated.

Issue-Id: SCM-5876
Signed-off-by: Yanqin Wei <Yanqin.Wei@arm.com>
Change-Id: If898a508dab1b27f6631ec70d4f1120f2fe44c96