Revise SD support
- Configure second SD controller to drive SDIO WiFi module and report via a HAL device
- Ensure in PostPowerOn that we don't try to set an unsupported voltage
- No need to dynamically select GetWriteProtect function
- Enable background operations
Requires RiscOS/BuildSys!47 (merged) and RiscOS/Sources/HWSupport/SD/SDIODriver!6 (merged) in order to build.
Requires Support/CI_Source!14 (merged) and Support/CI_Source!16 (merged) in order to pass CI.