cmd: pxe_utils: Check fdtcontroladdr in label_boot

If using OF_CONTROL, fdtcontroladdr is set to the fdt used to configure
U-Boot. When using PXE, if no fdt is defined in the menu file, and
there is no fdt at fdt_addr, add fall back on fdtcontroladdr too.

We are developing board support for the Armv8r64 FVP using
config_distro_bootcmd. We are also using OF_BOARD and would like the
PXE boot option to default to the fdt provided by board_fdt_blob_setup.

Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Ramon Fried <rfried.dev@gmail.com>