This will allow to link compiled object to the built-in-pbl.o across the source
tree that will be finally link to the pbl.
Now we compile the source %.c in pbl-%.o and provide -D__PBL__
so we can known in the source when it's compile for barebox or the pbl.
Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>