PicoCalc "southbridge" custom BIOS

This is my personnal rewrite of the original PicoCalc STM32F103R8T6 firmware.

The main differences with the original are followings:

• drastic reduction in the STM32's electricity consumption when running (~3.5 mA),
• removed stm32duino dependencies (use STM32HAL instead, maybe I'll switch to libopencm3 someday...),
• clean up (in progress) to reduce binary size (~25 KB) and allow more features to be implemented,
• added configuration saving solution (using internal flash, including backlight option),
• rewriten or added some debug UART interface message,
• lighten AXP2101 PMIC driver.

This source code can be compiled using ARM gcc toolchain (using v13) and make program.