Use the usart-common base plus the usart-v2 code, instead of private
implementations. Less code, more common apis across targets.
Of note is the trick to make F0 look like it has an APB2 bus. It's the
only stm32 that doesn't have a documented APB2 bus, but still has
peripherals enabled via an "APB2" register, and they match how other
targets have an APB2. Simply make APB2 an alias of APB1, as it's only
used for clock speed detection.
It was never complete, even for F1 family code, and went on to be even
less complete for f0 and f3. The usefulness of a library function to
check for both the irq being enabled _and_ the status flag is highly
questionable, and caused known user confusion.
The existing, much simpler, and fully functional usart_get_flag() is
a good replacement in almost all sane use cases.
Fixes https://github.com/libopencm3/libopencm3/issues/734
Rename rcc_ppre1_frequency and rcc_ppre2_frequency to rcc_apb1_frequency and rcc_apb2_frequency
Also add rcc_ahb_frequency (although it is not set correctly in all cases) which will be fixed by
the rcc commits later. Also fixup the only use in the library of these variables, the USART code.
And fix the typos that resulted
Make l1 generic too
