Add the option for "mpconfigboard.mk" to define whether the
board hosts a bootloader or not. The BOOTLOADER make variable
must be set to the name of the bootloader.
When the BOOTLOADER name is set it is also required to supply
the BOOTLOADER_VERSION_MAJOR and the BOOTLOADER_VERSION_MINOR
from the "mpconfigboards.mk". These will be used to resolve which
bootloader linker script that should be passed to the linker.
The BOOTLOADER section also supplies the C-compiler with
BOOTLOADER_<bootloader name>=<version major><version minor>
as a compiler define. This is for future use in case a bootloader
needs to do modification to the startup files or similar (like
setting the VTOR specific to a version of a bootloader).
718397a37d