Make mainboard_ops and mainboard.c file optional

This provides weak empty declaration for mainboard_ops.
The struct chip_operations is not defined for __PRE_RAM__ so
the declaration is also moved upwards in the output.

Change-Id: I101f0b8b9f0a55fb51a7c6475d53cc588c84026d
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: http://review.coreboot.org/1931
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
2 files changed