Code lineage:
osdebug() is based loosely on the version in esp8266, but there didn't
seem to be an obvious way of choosing a particular UART. The basic
behavior is the same, though: provide None, and logging is disabled;
provide an integer and logging is restored to the default level.

To build on that, and because the IDF provides more functionality, a
second parameter has now been implemented which allows the active log
level to be set:

   esp.osdebug(uart[, level])

The module has a corresponding set of LOG_ values to set this accordingly.