Minor fix for bios emulator makefile

Add $(obj) to LIB avoiding objects be built in the source dir
Signed-off-by: NJason Jin <Jason.jin@freescale.com>

drivers/bios_emulator/Makefile

 include $(TOPDIR)/config.mk
 
-LIB := libatibiosemu.a
+LIB := $(obj)libatibiosemu.a
 
 X86DIR  = ./x86emu