Fix 'expected str instance, bytes found' error when compute patches

When building an incremental OTA package, if the compress_target=True
parameter is passed in when calculating a block, an exception will occur
in the subsequent join operation.

The reason is that Python 3 no longer allows str type join bytes type.

Bug: None
Test: Build Incremental OTA package

Change-Id: I6d556f9905b7ab75b70d3785334d71d5a6e5479b
Signed-off-by: luoqiangwei1 <luoqiangwei1@xiaomi.com>