…s feedback

  Add MCUBoot v2 binary
  Add default keys to be flashed updating the loader

  Access libbootutil functions
  confirmSketch example

… sketches

…ings)

  build.keys.path    -> build.keys.keychain
  build.sign.name    -> build.keys.sign_key
  build.encrypt.name -> build.keys.encrypt_key

It also provides a way to change the keychain layout (if needed in the
future) but keep the backward compatiblity.

…tion"

This reverts commit 16810f58fe4d381a2cbc0a19da8d304fce93f56f.

At the moment this is not needed since we support only a key type. We could add it in later if a real need arises for other key types.

The property had the same form as the special `tools.TOOL_NAME.ACTION.pattern` properties
However, there is not a `build` action, the form of the property gives the impression that it is one that has special treatment by the build system.

It looks like the convention is `*.flags`

  Add applyUpdate(int permanent) to mark an update image as pending
  Add bootDebug(int enable) To enable/disable boot debug serial prints

  Fix Sketch header
  Use 100ms blink time
  Fix typo in comments

  bootloader binary; STM32H747_updateBootloader.ino
  libbootutil precompiled library