dropbear/openssh: Lower priority of key generation

Where we have images with PAM+systemd, serial login can be extremely
slow. The load generated by key generation does slow down the rest
of the boot process.

Lower the priority level of these systemd services, since we'd
prefer to have the rest of the system boot more effectively.

This doesn't "solve" the slow systemd boot issues but does help.

(From OE-Core rev: 087700665284c08ba846e52b6b86276629f5f1cd)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-connectivity/openssh/openssh/sshdgenkeys.service

@@ -6,3 +6,4 @@ RequiresMountsFor=/var /run
 ExecStart=@LIBEXECDIR@/sshd_check_keys
 Type=oneshot
 RemainAfterExit=yes
+Nice=10

meta/recipes-core/dropbear/dropbear/dropbearkey.service

@@ -11,3 +11,4 @@ Type=oneshot
 ExecStart=@BASE_BINDIR@/mkdir -p ${DROPBEAR_RSAKEY_DIR}
 ExecStart=@SBINDIR@/dropbearkey -t rsa -f ${DROPBEAR_RSAKEY_DIR}/dropbear_rsa_host_key
 RemainAfterExit=yes
+Nice=10