[FIXME] Resurrected, shinier USART support.

FIXME:
- Test F1 support
- Solve problem of duplicated bytes being TXed unless delay is
  inserted after configuration but before first bytes are TXed.

Rip out nonportable bits from top-level interfaces. The USART register
maps are basically the same between F1 and F2, so leave these, but add
register bit definitions which had name changes to the libmaple header
to avoid needless repetition. There are also a few new bits in the F2
USART registers; add definitions for these in the F2 USART header. Add
Doxygen comments for all USART bit definitions.

Deprecate struct usart_dev's max_baud field. This is just bloat that
doesn't bring us much real benefit.

Add new series-specific USART files for F1 and F2:

- libmaple/stm32f[1,2]/usart.c
- libmaple/stm32f[1,2]/include/series/usart.h

These are standard series-specific files, providing register map base
pointers, defining devices, implementing nonportable routines, etc.

We need a portable way to configure the USART GPIOs. To this end, add
usart_async_gpio_cfg() to the top-level USART interface. This function
is implemented in new F1 and F2 USART backends to take the appropriate
action to configure the RX and TX pins for asynchronous full duplex
mode.

USART baud rate calculation is done differently on the different
series. Keep the usart_set_baud_rate() declaration in the top-level
USART header, but move the implementations into the series-specific
usart.c files.

In usart_set_baud_rate(), allow for deriving clock_speed automatically
by letting user tell us to figure out the peripheral clock speed by
mapping the device's rcc_clk_id onto an STM32_PCLK[1,2] value. This
preserves flexibility for users with non-default clock configurations,
but makes things easier on everyone else.

Add private USART files for portable private USART routines:

- libmaple/usart_private.h
- libmaple/usart_private.c

Signed-off-by: Marti Bolivar <mbolivar@leaflabs.com>

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