Edit the /etc/exports file with the following settings. Note that these settings are not very secure, but they are necessary in order to have the system update files with root permission. Clients can't do any real damage to the server, but they can create files owned by root in the /srv directory tree.
/srv/tftpboot *(rw,insecure,no_root_squash,no_subtree_check) /srv/rootfs *(rw,insecure,no_root_squash,no_subtree_check)
If you edit the /etc/exports file you will need to tell NFS to reload the exported directories:
Check that NFS is running: