ARM: OMAP: Add CONFIG_OMAP_COMMON

Adding a new CONFIG_OMAP_COMMON which is included by all boards
that needs to build cpu/armv7/omap-common folder.

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
diff --git a/Makefile b/Makefile
index 7206aba..5461a21 100644
--- a/Makefile
+++ b/Makefile
@@ -322,7 +322,7 @@
 LIBS-y += post/libpost.o
 LIBS-y += test/libtest.o
 
-ifneq ($(CONFIG_AM33XX)$(CONFIG_OMAP34XX)$(CONFIG_OMAP44XX)$(CONFIG_OMAP54XX)$(CONFIG_TI814X),)
+ifneq ($(CONFIG_OMAP_COMMON),)
 LIBS-y += $(CPUDIR)/omap-common/libomap-common.o
 endif