Merge lp://qastaging/~milo/linaro-image-tools/default-bootloader into lp://qastaging/linaro-image-tools/11.11

This branch sets the default bootloader in a slightly different way than it was done before.
There is a small change in how two methods are called, and how a Config object is created.

In the code, we are using the Config object to create the real config when running linaro-hwpack-create (to create an hwpack) and also when running linaro-media-create (we are using the Config class to read the metadata file treating it as a Config object).

To maintain a little bit of compatibility with what was done before, now the Config object will call the set_board and set_bootloader methods only when the two parameters passed to the constuctor are defined (not None). Since when we are creating the hwpack with linaro-hwpack-create we are not interested in which bootloaders are present, but just if they are valid, we do not need to go through the setting of boards and bootloaders at that time. When we use linaro-media-create instead, we explicitly call set_board and set_bootloader, after validating the Config, in order to set the correct values.