pxe: Fix pxe boot with FIT image

When FIT image is used, a single image provides kernel, device
tree and optionally ramdisk. Argc and argv need to be adjusted
to support this.

Test cases:
	1. Booting with legacy images
	2. Booting with legacy images without initrd
	3. Booting with FIT image
Test commands:
	1. pxe get && pxe boot
	2. sysboot

Signed-off-by: York Sun <york.sun@nxp.com>
Signed-off-by: Wenbin Song <wenbin.song@nxp.com>