boot: pxe_utils: skip fdt setup in case legacy kernel is booted

boot: pxe_utils: skip fdt setup in case legacy kernel is booted

Currently, if boot with extlinux.conf and do not set the fdt
U-Boot will provide its own device tree. This behavior is
beneficial if the U-Boot device tree is in sync with Linux,
but it totally halts the booting of pre-dtb kernels (3.4 for
example) since it uses ATAGs. To fix this, pass `-` in the
fdt extlinux field as a signal that no tree should be used.

Suggested-by: Jonas Schwöbel <jonasschwoebel@yahoo.de>
Tested-by: Jethro Bull <jethrob@hotmail.com>
Signed-off-by: Svyatoslav Ryhel <clamor95@gmail.com>