cmd: pxe: reorder kernel treatment in label_boot

Reorder kernel treatment in label_boot at the beginning of the function.

This patch doesn't change the pxe command behavior, it is only a
preliminary step for next patch, build kernel_addr before parsing
the label initrd and fdt to build the next bootm arguments.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>