hrcon: Migrate to CONFIG_TARGET_HRCON

Use the proper CONFIG_TARGET_HRCON Kconfig option to replace
the CONFIG_HRCON ad-hoc config option.

Signed-off-by: Mario Six <mario.six@gdsys.cc>
diff --git a/board/gdsys/mpc8308/Makefile b/board/gdsys/mpc8308/Makefile
index 60d2232..f29376f 100644
--- a/board/gdsys/mpc8308/Makefile
+++ b/board/gdsys/mpc8308/Makefile
@@ -4,5 +4,5 @@
 # Dirk Eibach,  Guntermann & Drunck GmbH, dirk.eibach@gdsys.cc
 
 obj-y := mpc8308.o sdram.o
-obj-$(CONFIG_HRCON) += hrcon.o
+obj-$(CONFIG_TARGET_HRCON) += hrcon.o
 obj-$(CONFIG_STRIDER) += strider.o