doc: improve flash-write example
Page 5 of the manual states that the SDBFS can be written to file, as
usual the program "tools/flash-write" that does this has no help (-h
not implemented).
The command given in the manual fails, it turns out that the user has
to think that the bus -b 0x20 needs to be taken from the dmesg output
(given a bit further on the page) and is apparently the number between
the first and second semicolon in the dmesg lines that are printed
when the kernel drivers is inserted.
So two things:
- - add help to flash-write tool
- - clarify where the user should find the argument for the -b option.