defaultenv: don't overwrite $user in config

This patch prooved to be useful while working with a colleague on a single
machine. It allows to switch the user without editing /env/config by
just issuing:

	export user=me

. Note, in bash this construct (i.e. an if without body) bails out with:

	Syntax error: "fi" unexpected

but in hush it seems to do the right thing.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

defaultenv/config

@@ -1,7 +1,9 @@
 #!/bin/sh
 
 hostname=FIXME
-#user=
+if [ -z "$user" ]; then
+#	user=
+fi
 
 # Enter MAC address here if not retrieved automatically
 #eth0.ethaddr=de:ad:be:ef:00:00