[coreboot] LegacyBIOS -> SeaBIOS in buildrom

[Myles Watson]

This patch changes the name of LegacyBIOS to SeaBIOS, everywhere but the git
URL, which hasn't been updated yet.  It also updates the hardcode.diff patch
now that the memory value is read from the coreboot tables, and fixes the
handling of that patch.

Thanks,
Myles

Signed-off-by: Myles Watson