stm32f1x_cl: Add initial support for STM32F1x connectivity line

This currently supports the stm32f107, but support will be added for
the F105 shortly (missing a linker script and I can't properly test
without a devkit).
stm32f107_basic: Add support for a simple stm32f107 platform

This platform is a basic waveshare stm32f107 devkit which contains a
USART, USB device port, some buttons and some LEDs.  Unfortunately not
enough to bring up networking, but enough to test building and a
simple contiki shell