minor changes to make stm32f103 code work

[stefaandesmet2003]

Hi,
great work on this repo! I had to make 2 minor changes to make the code work on bluepill :

• generic/config.h : misses #define UF2_FAMILY 0x5ee21072
• backup.c : not sure what the reason was not to use the BKP registers, but (void)reg;
  (volatile uint32_t)0x2000f000 = value; didn't work on bluepill, because it's addressing non-existing RAM (only 20k), and panics the cortex core. I used the version from devanlai's repo, and all good!

Cheers,

Stefaan

[yuanyanhui]

@stefaandesmet2003 Does your bluepill work properly with makecode? I tried the uf2 release in the repo as well as a new build according to your changes, however in both cases codes compiled by makecode (maker.makecode.com) cannot run on my board.