[linker] Expand and correct symbol requirement macros
REQUIRE_SYMBOL() formerly used a formulation of symbol requirement
that would allow a link to succeed despite lacking a required symbol,
because it did not introduce any relocations. Fix by renaming it to
REQUEST_SYMBOL() (since the soft-requirement behavior can be useful)
and add a REQUIRE_SYMBOL() that truly requires.
Add EXPORT_SYMBOL() and IMPORT_SYMBOL() for REQUEST_SYMBOL()-like
behavior that allows one to make use of the symbol, by combining a
weak external on the symbol itself with a REQUEST_SYMBOL() of a second
symbol.
Signed-off-by: Marty Connor <mdc@etherboot.org>
Prevent NULL from ever pointing to some real code or data. (This was
causing the serial console to ignore input, because it happened to end up
linked with serial_ischar() at address 0, which core/console.c decided was
invalid).