Rework main board power input and voltage regulation

[dkonigsberg]

The main board receives +12VDC from the power board, from which it then needs to provide both 3.3V and 5V rails for various purposes. The on-board components typically run off 3.3V, while the front ports (including USB) require 5V to operate.

The current design brings power into a pair of RT8258 buck regulator circuits, from which both rails are generated.

The proposed change is to do the following:

• Redesign the power input path using some suggestions from Infineon AN80994, such that both the +12VDC and GND inputs pass through various components before going to the buck regulators or ground plane, to improve power quality and hopefully reduce the impact of transients.
• Replace the current buck regulator setup with one based around the TI TPS54294 dual converter, which has performed quite well in testing and should make it easier to use a single input filter path
• Clean up the output path, removing unnecessary components (the ferrite beads currently there are likely unnecessary, for example)

The motivation behind these changes is similar to the motivation behind #56, #60, and #65, which is to generally improve the power subsystem in the device. Actual cost is unlikely to change much, and there may be a slight reduction in component count.

[dkonigsberg]

Changes implemented on the Rev E hardware.