[gta02-core] ECN0010 re-opened (was Re: More audio considerations)

[Werner Almesberger]

Since we still have to clean up the power supply, I reopened ECN0010
and added a summary of the power supply discussion. The broad strokes
are as follows:

- swap CODEC and BT power, so that we have 150 mA for the amplifiers
  (instead of 50 mA)

- consider feeding LDOs 3/4 (IO_2V8 and CODEC_3V3) from IO_3V3 instead
  of Vsys, for better regulation efficiency

Comments, particularly about the latter issue ?

- Werner