[coreboot] [PATCH]libpayload: stub USB functions, if not compiled in

[Patrick Georgi]

Hi,

some people had issues with libpayload without USB features, and payloads
failing to build because some functions were missing.
This patch adds stubs in case USB is not selected.

It's mostly a temporary measure, until there's some proper mechanism to
figure out how libpayload was configured, and it probably shouldn't get
into the official tree at all, but I won't decide that one. :-)

Regards,
Patrick