[stm32] stm32f1xx: initial support for stm32f1xx - infrastructure.
Linker script is configured for STM32F103RBT
[stm32] stm32f103rbt - add CONFIG
[stm32] add armjishu 28" board support
[stm32] stm32f1xx boards do not seem to require AF set on RX USART pins