[STM32F051R8] flash 64K, sram 8k, 64pin
define STM32F051
usart1 tx,rx = PA9, PA10 (BT BM57SPP03)
usart2 tx,rx = PA2,PA3 (usart debug)
linker command line  : --keep __fsym_* --keep __vsym_*
TODO : stm32_rom.ld for smaller flash and sram
STM32F050 has only 4K SRAM with 16,32,64K flash