[coreboot] Conflicting default options - option ROM exec and SeaBIOS

[Alex G.]

Right now, the default options in Kconfig are to pull SeaBIOS stable,
AND enable option ROM execution. This leaves people with a system that
has option ROMs executing twice.  As you might have imagined, this
results in undefined behavior. Some people are lucky and their systems
still run, while others,...

Default options should be changed to disable option ROM execution when
SeaBIOS is built in-tree.

Alex