naively adding UF2 blocks together is at least wrong by spec; it
produces one file with wrong block counts --- say I'm writing 3770
blocks, the naive uf2(firmware) + uf2(config) solution yields a file
that says it's 3642 blocks for a while, then 128 blocks. picotool kind
of does a "wtf" at this but writes it anyway, but I am not confident
this is safe, so I'll just do the dumb thing again for now (meaning we
pretty much always write 8128 or 8192 blocks when concatenating configs)
Signed-off-by: Brian S. Stephan <bss@incorporeal.org>