Unify stm32-specific USART configurations by abstracting serial/uart operations

[iankuan]

Aim to solve #9 and improve serial abstraction layer for more clear .

Current serial impl isn't good enough to separate generic mechanism and platform device impl well. Here are pieces of evidence.

1. Improper functions which couldn't convey what it does via function names, such as stm32f1xx_init().
2. No generic serial function initialization routine in serialcore.c

The inclusion of this patch:

• Refactor current serial impl
• Restructure serial abstraction layer

[jserv]

Is it possible to consolidate existing stm32f1 and stm32f4 uart implementation into single one?

[jserv]

Rebase is required.