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/avnet/fx12mm/Makefile b/board/avnet/fx12mm/Makefile
index 2dd48b6..37fb02f 100644
--- a/board/avnet/fx12mm/Makefile
+++ b/board/avnet/fx12mm/Makefile
@@ -6,6 +6,6 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
-COBJS	+= $(BOARD).o
+obj-y	+= fx12mm.o
 
 include $(SRCTREE)/board/xilinx/ppc405-generic/Makefile