sys/config: Simplify conf_str_from_value()
sys/config: Simplify conf_str_from_value()

Don't treat 64-bit integers specially.  Just convert all integers to
64-bit and process them with common code.

mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core