Add post-install hints for boards without distro_bootcmd (like BPi F3/Jupiter)

The default bootcmd is useless on these boards, so suggest some simple
default boot command.

distrib/notes/riscv64/prep

@@ -1,4 +1,4 @@
-dnl	$OpenBSD: prep,v 1.8 2026/04/20 21:38:55 jca Exp $
+dnl	$OpenBSD: prep,v 1.9 2026/04/20 21:43:39 jca Exp $
 Please be aware that OpenBSD support for this platform is far from
 complete, however progress is being made.
 
@@ -34,6 +34,13 @@ script.
 The bootloader will then run and try to load sd0a:/bsd off an FFS
 filesystem after a timeout.
 
+After a successful install to an nvme drive, you can then teach U-Boot
+to look at the nvme drive by default.
+        => setenv custom_nvme_boot "load nvme 0:1 ${kernel_addr_r} \
+        efi/boot/bootriscv64.efi; bootefi ${kernel_addr_r}"
+        => setenv bootcmd "run custom_nvme_boot"
+        => saveenv
+
 HiFive Unmatched
 
 Copy install{:--:}OSrev.img to a USB stick, and boot with both it and the