Makefile: convert makefiles to Kbuild style and delete grep switch

We have converted all makefiles needed to build $(LIBS).

Until this commit we used to grep switch so that U-Boot style
and Kbuild style makefiles coexist.
But we do not need any more.

Goint forward, use always Kbuild style Makefile when adding
a new Makefile

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/board/nvidia/common/common.mk b/board/nvidia/common/common.mk
index d9bcb85..9a9b529 100644
--- a/board/nvidia/common/common.mk
+++ b/board/nvidia/common/common.mk
@@ -1,3 +1,3 @@
 # common options for all tegra boards
-COBJS-y	+= ../../nvidia/common/board.o
-COBJS-$(CONFIG_TEGRA_CLOCK_SCALING) += ../../nvidia/common/emc.o
+obj-y	+= ../../nvidia/common/board.o
+obj-$(CONFIG_TEGRA_CLOCK_SCALING) += ../../nvidia/common/emc.o