Whan add some obj in a subdir to lwl-y or pbl-y, like this:

     lwl-y += subdir/test.o other.o

the make process failed:

     make[2]: *** No rule to make target
'arch/arm/boards/boardname/test.o', \
         needed by 'arch/arm/boards/boardname/built-in-pbl.o'.  Stop.

Note, there are not the part "subdir" in the path of the test.o.

this patch fix this

Signed-off-by: 张忠山 <zzs213@126.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>