kbuild: do not use $(BOARD) to specify exact object name

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/board/broadcom/bcm28155_ap/Makefile b/board/broadcom/bcm28155_ap/Makefile
index b18785a..4bb9e70 100644
--- a/board/broadcom/bcm28155_ap/Makefile
+++ b/board/broadcom/bcm28155_ap/Makefile
@@ -4,4 +4,4 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
-obj-y	+= $(BOARD).o
+obj-y	+= bcm28155_ap.o