android: mkbootimg: allow dtb.img to be optional

Signed-off-by: hmz007 <hmz007@gmail.com>

system/tools/mkbootimg/mkbootimg.py

@@ -131,7 +131,7 @@ def write_header_v3_and_above(args):
 
 
 def write_vendor_boot_header(args):
-    if filesize(args.dtb) == 0:
+    if filesize(args.dtb) == 0 and filesize(args.second) == 0:
         raise ValueError('DTB image must not be empty.')
 
     if args.header_version > 3:
@@ -252,7 +252,7 @@ def write_header(args):
         args.output.write(pack('I', BOOT_IMAGE_HEADER_V2_SIZE))
 
     if args.header_version > 1:
-        if filesize(args.dtb) == 0:
+        if filesize(args.dtb) == 0 and filesize(args.second) == 0:
             raise ValueError('DTB image must not be empty.')
 
         # dtb size in bytes