Initial clone of other/i2c_stts75_sensor

Name changed and LD script fixed up only.
Fix readme to be more relevant
Add vl disco defines to for "make flash"
printf and syscfg stub, now for the meat of i2c :)
WIP: failing to reliably make i2c work
l1-sht21: stub import of cs43l22 i2c code
i2c-sht21-wip: turn off acks and aborted writes for sht21

Now it reads the user register and decodes it's values.

As opposed to reading out of the cs43l22, a write is always one byte command,
so no stop afterwards, and turn off the acks.
l1: sht21: needs tidy/squish: working single function to i2creadx

thanks jadew on irc, who is infuriating in his documentation twisted
interpretation skills.
i2c-sht21 add reading the serial number.