The configuration bits for the UART register were wrong and the parity
couldn't be enabled, because the exist_parity member hasn't been updated. I
took this ESP8266 register description (http://esp8266.ru/esp8266-uart-reg/)
as reference.
Verification has been done with a logic analyzer.
These symbols are still defined in terms of the system Exxx symbols, and
can be switched to internal numeric definitions at a later stage.
Note that extmod/modlwip still uses many system Exxx symbols.