drivers: descend into sub directories only when it is necessary

- Descend into drivers/fpga/ only when CONFIG_FPGA=y
  - Descend into drivers/bios_emulator only when CONFIG_BIOSEMU=y

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/drivers/bios_emulator/Makefile b/drivers/bios_emulator/Makefile
index dd42e0f..9aa8ea9 100644
--- a/drivers/bios_emulator/Makefile
+++ b/drivers/bios_emulator/Makefile
@@ -2,7 +2,7 @@
 
 $(shell mkdir -p $(obj)$(X86DIR))
 
-obj-$(CONFIG_BIOSEMU)	= atibios.o biosemu.o besys.o bios.o \
+obj-y = atibios.o biosemu.o besys.o bios.o \
 	$(X86DIR)/decode.o \
 	$(X86DIR)/ops2.o \
 	$(X86DIR)/ops.o \