Minimal clock mux config

[kaolpr]

A minimal clocking mux configuration. Please note that we can't put clock_switch_default_config() in board_config() as there is no power supply for P3V3 at that point.

[kaolpr]

Additionally, this dangerous loop around writing those registers is related to #112. Maybe we could setup some error-handling macro?

[augustofg]

Yes we've identified this issue with the P3V3 before and corrected it in our 'dirty' local branch, but your code seems cleaner. I'll do some checks here before merging.

Error handling is a problem with openMMC in general, I'm drafting a proposal to make it consistent (error return enum, error codes, error strings etc). From our tests the IDT 8V54816 worked reliably every time at least.

[augustofg]

You should declare uint8_t clock_switch_default_config(void) in the board_config.h header to avoid compiler warnings.

[augustofg]

You also should include board_config.hin the payload.c.

[augustofg]

I'm closing this due to #165. Now the clock mux configuration is done via IPMI and saved in the RTC EEPROM.