The bootloader settings are handled through attributes in the
type element. Over the years some attributes were added and
there are requests for more settings. Therefore the bootloader
setup deservers its own section. With this commit the schema
changes from v7.1 to v7.2 and moves bootloader, bootloader_console
boottimeout and zipl_targettype into a new bootloader subsection
below type. The commit also adds an auto transformation template
such that customers don't have to change their image descriptions.
This is related to Issue #1401
With the move of the boot descriptions in its own package
the kiwi search path needs to be adapted to look for boot
descriptions in /usr/share/kiwi/custom_boot. In addition
a runtime check to exit early if the boot description could
not be found or is not specified will be provided. Related
to Issue #576
