bootm: use genimg_get_kernel_addr()

Use the new API which is originally taken out from boot_get_kernel
of bootm.c

Signed-off-by: Bryan Wu <pengw@nvidia.com>
Tested-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
[trini: Fix warnings with CONFIG_FIT]
Signed-off-by: Tom Rini <trini@ti.com>