sys: add riotboot_hdr submodule

riotboot_hdr enables to partition the internal flash memory
into "slots", each one with a header providing information
about the partition. The concept for now is limited to
firmware partitions, which are recognised by the riotboot
bootloader. In the future the concept might be extended to
represent other content.

Co-authored-by: Kaspar Schleiser <kaspar@schleiser.de>