platform/stm32-e407/: copy of most of platform/stm32ldiscovery/
remove dev/uart-stm32-l1.c

Better to extend the original.
stm32ldiscovery/dev/uart-stm32-l1.c: add UART6 (for sharing with STM32F2/F4)
platform/stm32-e407/: changes for Olimex STM32-E407 (WIP)
platform/stm32-e407/: add links to rtimer-arch.[ch]