New system overlay technique about openbmc

[tfangit]

Ideally, board specific changes are in meta-openbmc/meta-company/meta-board. It is achievable but adding difficulty for development and maintenance, specially, in u-boot and kernel. Both u-boot and kernel already have great mechanism to support different boards/HWs. We would like leverage such mechanism, which means, HW/boards specific changes will be in SoC layer.

u-boot has board layers, currently, we re-used the ast2400 board layer for wedge development. That won't work for different boards. We will need to create separate directory for each different boards.

In kernel, we created separate configuration for wedge, wedge_defconfig. Then, use '#ifdef CONFIG_WEDGE' for wedge specific changes. However, we want to limit usage of '#ifdef' in source code. For example, for the code to enable/disable LPC, I would prefer to having a macro like CONFIG_ASPEED_ENABLE_LPC used in the source code, then each different HW defconfig has that macro defined or not depending on board specific design.

[causten]

Sounds good, I'll get you some changes that adhere to the concepts. Employing the _defconfig more fits with our thoughts too