Fix reinstall brick

[xyzz]

Don't run this unless you can recover from a brick.

Situation: user installs enso, then reinstalls 3.60 (which uninstalls enso but not the backup MBR), then runs enso app and selects uninstall. Result: brick.

This fixes it by always overwriting block 0 with a copy of block 0 which when enso is installed would redirect the reads to block 1 but not writes, and when enso is not installed would not do anything.

Tests:

• [x] Without the patch: Install enso, reboot, uninstall enso
• [x] Without the patch: Install enso, flash 3.60, make nand backup !!, uninstall enso; confirm this bricks
• [x] With the patch: Install enso, flash 3.60, make nand backup !!, uninstall enso; confirm this does not brick anymore
• [x] With the patch: Install enso, reboot, uninstall enso
• [x] With the patch: Install enso, reboot, uninstall enso, reboot, uninstall enso
• Run all tests from the old google forms

make a nand backup every time you reflash 3.60!

[yifanlu]

Confirmed that original enso bricks with indicated steps. Confirmed patch fixes the problem. Because the mbr.bin payload is exactly the same, I did not feel the need to run the old tests.