Symbol export whitelist

Symbol export whitelist

Before modules were dynamically loaded the boundary between GPL and
non-GPL code was implicit because of the separate link domains for
each module. With dynamic modules we need an explicit whitelist of
core symbols that non-GPL code can link against at runtime without
needing to be re-licensed under the GPL.

Mark such symbols with __export, so that it is explicitly clear which
symbols in the core can be linked against by non-GPL code.

Reduce the visibility of symbols in both the core and ldlinux.c32 with
-fvisibility=hidden. __export changes the visibility to 'default'.

Signed-off-by: Matt Fleming <matt.fleming@intel.com>