QcomModulePkg: BootImagev3: Update DTB offsets

For boot-image header version three, the DTB is packaged as
a part of vendor-boot image. Hence, update the image buffer,
DTB size and offset accordingly to get the device tree and
parse it.

Also, replace the variable DtbSize with ImageSize in
DTBImgCheckAndAppendDT() as it's a little misleading.
Technically, it's used to hold the size of the Image
so that the DT can be extracted properly from it.

Change-Id: I19cb04ce60f00968baebc7e12adadb8dbad68b04
diff --git a/QcomModulePkg/Library/BootLib/LocateDeviceTree.c b/QcomModulePkg/Library/BootLib/LocateDeviceTree.c
index 43efc78..ce6df08 100644
--- a/QcomModulePkg/Library/BootLib/LocateDeviceTree.c
+++ b/QcomModulePkg/Library/BootLib/LocateDeviceTree.c
@@ -914,6 +914,8 @@
 /*
  * For Header Version 2, the arguments Kernel and KernelSize will be
  * the entire bootimage and the bootimage size.
+ * For Header Version 3, Kernel holds the base of the vendor-boot
+ * image and KernelSize holds its size.
  */
 VOID *
 GetSocDtb (VOID *Kernel, UINT32 KernelSize, UINT32 DtbOffset, VOID *DtbLoadAddr)