cmd: pxe: add alias devicetree-overlay for fdtoverlays

This adds keyword devicetree-overlay as an alias for fdtoverlays in
extlinux (sysboot) and pxe to better follow the Boot Loader Specification
[1], improves documentation around them by adding an example for both
fdtoverlays and devicetree-overlay and the environment variable required
for this feature. The link for the spec is updated to the current one.

[1] https://systemd.io/BOOT_LOADER_SPECIFICATION/

Signed-off-by: Edoardo Tomelleri <e.tomell@gmail.com>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>

boot/pxe_utils.c

@@ -380,6 +380,7 @@ err:
 
 /**
  * label_boot_fdtoverlay() - Loads fdt overlays specified in 'fdtoverlays'
+ * or 'devicetree-overlay'
  *
  * @ctx: PXE context
  * @label: Label to process
@@ -809,6 +810,7 @@ static const struct token keywords[] = {
 	{"devicetreedir", T_FDTDIR},
 	{"fdtdir", T_FDTDIR},
 	{"fdtoverlays", T_FDTOVERLAYS},
+	{"devicetree-overlay", T_FDTOVERLAYS},
 	{"ontimeout", T_ONTIMEOUT,},
 	{"ipappend", T_IPAPPEND,},
 	{"background", T_BACKGROUND,},