cubieboard: mkcubiecard: use the right command for checking user privilege

On some shell environments the EUID variable, which checks the
user id, is not defined. Use `id -u` instead.

This patch mutes the following error:

./board/cubietech/cubieboard/mkcubiecard.sh: 31: [: -ne: unexpected operator

Signed-off-by: Andi Shyti <andi@etezian.org>
Reviewed-by: Arvid Picciani <aep@exys.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>