fix alignment bug on erase blocks for usb_update.py

tools/README_UPDATE.md

@@ -11,8 +11,16 @@ Maintainer's note: this is a mirror of the README.md file found in [betrusted-ec
 * `sudo pip3 install pyusb progressbar2` (you need `pyusb` and `progressbar2` in the sudo environment)
 * you probably need to run as `sudo`: `sudo ./usb_update.py [args]` to access the USB port; add the appropriate command line arguments
 
-If you don't like/can't use `sudo`, I hear something involving udev can solve this problem, but I've never been able to get it to work. If someone can contribute a simple, robust cross-distro guide to USB `udev` rules that ages well, I'm guessing the Internet would appreciate it.
+In order to get `xtask` to work (it calls `usb_update.py`) you will need to add a udev rule to eliminate the need for sudo privs. Under
+Ubuntu 18.04LTS, it looks something like this:
 
+1. Add a file called `99-precursor-usb.rules` to /etc/udev/rules.d/ with the following content as a single line terminated by a carriage return:
+`SUBSYSTEM=="usb", ATTRS{idVendor}=="1209", ATTRS{idProduct}=="5bf0", GROUP="plugdev", TAG+="uaccess"`
+2. Run `sudo udevadm control --reload-rules`
+3. If the device was plugged in, you'll have to unplug it and plug it back in again for the new rules
+to take effect.
+
+We would appreciate any improvements/suggestions for alternate Linux distros. `udev` rules seem very distro-specific and change with time.
 ### Installation: Windows 10
 
 * You will need to install python3 from the Windows App store

tools/usb_update.py

@@ -253,7 +253,7 @@ class PrecursorUsb:
         erased = 0
         while erased < len(data):
             self.ping_wdt()
-            if len(data) - erased >= 65536:
+            if (len(data) - erased >= 65536) and ((addr & 0xFFFF) == 0):
                 blocksize = 65536
             else:
                 blocksize = 4096
@@ -264,7 +264,7 @@ class PrecursorUsb:
                 if status & 0x02 != 0:
                     break
 
-            if blocksize <= 4096:
+            if blocksize == 4096:
                 self.flash_se4b(addr + erased)
             else:
                 self.flash_be4b(addr + erased)