[BITBAKE] Handle custom Edgehog distros

In order to handle the scenarios where bitbake needs to compile an
image different from the standard ones we introduce the possibility to
have a custom DISTRO variable value. However, the variable shall follow
the following convention:

* The first part of the string shall indicate the standard Edgehog OS
  distro, i.e. 'things' or 'embedded' as of now
* The second part of the string shall be the custom Edgehog OS image
  distro with at maximum a '-' as a separator, e.g. 'custom-image'
* The two parts of the string shall be separated by a '-'

An example could be 'embedded-custom-image'.