pxe: support "devicetree" tag

The specification for extlinux.conf[1] states that "fdt" is an alias for
"devicetree". To date, U-Boot only implements "fdt". Rectify that.

[1] http://freedesktop.org/wiki/Specifications/BootLoaderSpec/

Signed-off-by: Stephen Warren <swarren@nvidia.com>

common/cmd_pxe.c

@@ -745,6 +745,7 @@ static const struct token keywords[] = {
 	{"append", T_APPEND},
 	{"initrd", T_INITRD},
 	{"include", T_INCLUDE},
+	{"devicetree", T_FDT},
 	{"fdt", T_FDT},
 	{"ontimeout", T_ONTIMEOUT,},
 	{"ipappend", T_IPAPPEND,},