uboot-kirkwood: fix goflexhome/net bootcommand

Goflexhome/net use uImage, and to boot an uImage the u-boot
must use bootm command, not bootz.

Fixes the "i cannot boot LEDE with this u-boot" issue that I
found out myself with my goflexnet.

Signed-off-by: Alberto Bursi <alberto.bursi@outlook.it>