Add a section in style guide: __func__ should be used over __FUNCTION__.

Use booleans to represent boolean state, instead of a set of masks into an

integer. It's more transparent and readable, and less error prone.

### Function names as strings

C99 defines `__func__` as an identifier that represents the function's name

in which it is used. The magic identifier `__FUNCTION__` should not be used

as it is a non-standard language extension and an equivalent standardized

mechanism exists. In other words, use `__func__` over `__FUNCTION__`.

## Bluedroid conventions

This section describes coding conventions that are specific to Bluedroid.

Whereas the _Language_ section describes the use of language features, this