cmd: pxe: add support for FDT overlays

This adds support for specifying FDT overlays in an extlinux/pxelinux
configuration file.

Without this, there is no simple way to apply overlays when the kernel
and fdt is loaded by the pxe command.

This change adds the 'fdtoverlays' keyword for a label, supporting multiple
overlay files to be applied on top of the fdt specified in the 'fdt' or
'devicetree' keyword.

Example:
  label linux
    kernel /Image
    fdt /soc-board.dtb
    fdtoverlays /soc-board-function.dtbo
    append console=ttyS0,115200 root=/dev/mmcblk0p2 rootwait

This code makes usage of a new variable called fdtoverlay_addr_r used to load
the overlay files without overwritting anything important.

Cc: Tom Rini <trini@konsulko.com>
Cc: Andre Heider <a.heider@gmail.com>
Cc: Jernej Škrabec <jernej.skrabec@siol.net>
Cc: Jonas Karlman <jonas@kwiboo.se>
Tested-by: Jernej Škrabec <jernej.skrabec@siol.net>
Reviewed-by: Jernej Škrabec <jernej.skrabec@siol.net>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>