[Feature] Support STM32F103C6 (32KB Flash, 10KB RAM) on stm32f103-blue-pill #11854

Affected area

Board Support Package (BSP)

Hardware/BSP vendor

STM32

Architecture

ARM / AArch64

Describe problem solved by the proposed feature

Currently, bsp/stm32/stm32f103-blue-pill only supports the STM32F103C8 chip (64KB/128KB Flash, 20KB RAM).
However, many widely available Blue Pill development boards on the market come populated with the lower-cost, pin-compatible STM32F103C6T6 (32KB Flash, 10KB RAM).

Attempting to run the existing BSP on an STM32F103C6 board causes several issues:

1. Flash Overflow: The default firmware image exceeds 32KB, failing to fit or link for the C6 flash boundary.
2. RAM Corruption: board.h and the linker script assume 20KB SRAM (HEAP_END at 0x20005000), whereas C6 has only 10KB SRAM (HEAP_END at 0x20002800). Any heap allocation past 10KB causes silent memory corruption or HardFault.
3. Peripheral Differences: STM32F103C6 silicon physically lacks USART3 and SPI2.

Describe your preferred solution

Add support for the STM32F103C6 variant to bsp/stm32/stm32f103-blue-pill while keeping 100% backward compatibility for the default STM32F103C8:

1. Kconfig Chip Choice:

   • Add a choice between SOC_STM32F103C8 (default) and SOC_STM32F103C6.
   • When C6 is selected, automatically select RT_USING_SMALL_MEM, RT_USING_SMALL_MEM_AS_HEAP, and RT_KLIBC_USING_LIBC_VSSCANF.

2. Accurate Memory Boundaries (board.h & link.lds):

   • Dynamically define STM32_FLASH_SIZE (32KB) and STM32_SRAM_SIZE (10KB) under SOC_STM32F103C6 so HEAP_END is strictly bounded at 0x20002800.
   • Update link.lds with dynamic _ROM_SIZE and _RAM_SIZE symbols.
   • Pass -Os and --defsym=_ROM_SIZE=32k,--defsym=_RAM_SIZE=10k in rtconfig.py for C6 builds.

3. Peripheral Constraints (board/Kconfig):

   • Guard USART3, SPI2, and RTduino with depends on !SOC_STM32F103C6.
   • Add BSP_USING_SPI1 option (supported on C6).

4. Flash Optimization Guide:

   • Document optimization steps in README.md and README_zh.md (disabling serial DMA, FinSH symbol descriptions, and reducing shell stack to 2048) to fit standard RT-Thread with interactive FinSH (msh >) within 32KB.

Describe possible alternatives

I have already implemented and verified this solution in the PR #11850