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/Makefile b/drivers/Makefile
index 9cec2ba..95a4b80 100644
--- a/drivers/Makefile
+++ b/drivers/Makefile
@@ -1,8 +1,8 @@
-obj-y += bios_emulator/
+obj-$(CONFIG_BIOSEMU) += bios_emulator/
 obj-y += block/
 obj-$(CONFIG_BOOTCOUNT_LIMIT) += bootcount/
 obj-y += crypto/
-obj-y += fpga/
+obj-$(CONFIG_FPGA) += fpga/
 obj-y += hwmon/
 obj-y += misc/
 obj-y += pcmcia/