FLASHFILE is now set to RIOTBOOT_EXTENDED_BIN, changing the meaning of
make all`, `flash`, `flash-only`.
This commits updates the documentation accordingly.
riotboot is introduced here and makes use of riotboot_hdr,
which indentifies the images encapsulated as slots.
The slot size and offset is configurable, which makes
slots extendable if needed, e.g. 2 or more slots can be
transparently added.
Co-authored-by: Kaspar Schleiser <kaspar@schleiser.de>
Co-authored-by: Gaëtan Harter <gaetan.harter@fu-berlin.de>
