Fix reconstructed softraid bootline

Fix reconstructed softraid bootline

With softraid, OBP's boot-file variable aka. bootline may contain the
sofraid volume name and kernel file name delimited by a double colon, e.g.
"sr0", "sr0a", "sr0:", "sr0a:", "sr0:/bsd" or "sr0a:/bsd".

ofwboot parses this string, may fill in optional partition number ("a")
and/or optional kernel file name ("/bsd") and always prints the fully
qualified string reconstructed from parts as "Booting <bootline>":
	{0} ok boot my-devalias sr0
	[...]
	Boot device: /virtual-devices@100/channel-devices@200/disk@0  File and args: sr0
	[...]
	Booting sr0:a/bsd
	[...]

	{0} ok boot my-devalias sr0a:/bsd
	[...]
	Boot device: /virtual-devices@100/channel-devices@200/disk@0  File and args: sr0a:/bsd
	[...]
	Booting sr0:a/bsd
	[...]

Swap partition and delimiter to fix the reconstructed string, making it
suitable for copy/paste:
	Booting sr0a:/bsd

This has not been an issue since the reconstructed string is only printed
and not (re)used anywhere.

OK kettenis