board: stm32_min_dev: Add support for blue/black variant

Add support for blue pill and black pill variants of the STM32
minimum development board by splitting board configuration into
stm32_min_dev_blue and stm32_min_dev_black.

CODEOWNERS: Add myself (@cbsiddharth) as codeowner for stm32_min_dev

Signed-off-by: Siddharth Chandrasekaran <siddharth@embedjournal.com>