picosoc: fixed missing use of BOARD_BUILDIR
picosoc: fixed missing use of BOARD_BUILDIR

Signed-off-by: Rodrigo Alejandro Melo <9115257fd96058a415f7c09f9a45c6a6876e4da8@gmail.com>

SymbiFlow/symbiflow-examples,SymbiFlow/symbiflow-examples