Fix linking errors

Fixes the following linking errors on gcc 10:
/usr/bin/ld: libanyio.a(spi_boards.o):spi_boards.c:41:
 multiple definition of `sd';
 libanyio.a(eth_boards.o):eth_boards.c:49: first defined here
/usr/bin/ld: libanyio.a(serial_boards.o):serial_boards.c:38:
 multiple definition of `sd';
 libanyio.a(eth_boards.o):eth_boards.c:49: first defined here
/usr/bin/ld: libanyio.a(lbp.o):mesaflash/lbp.c:31:
 multiple definition of `sd';
 libanyio.a(eth_boards.o):eth_boards.c:49: first defined here

Signed-off-by: Damian Wrobel <dwrobel@ertelnet.rybnik.pl>