defaultenv/bin/boot: fix ubi root's name assignment
This patch fixes a regression introduced in commit
d15cfaf.
While there, use "-z" to check if ubiroot variable is empty.
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Acked-By: Eric Bénard<eric@eukrea.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>