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

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>

board/broadcom/bcm28155_ap/Makefile

@@ -4,4 +4,4 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
-obj-y	+= $(BOARD).o
+obj-y	+= bcm28155_ap.o

board/compulab/cm_t335/Makefile

@@ -6,5 +6,5 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
-obj-y	+= $(BOARD).o
+obj-y	+= cm_t335.o
 obj-$(CONFIG_SPL_BUILD) += mux.o spl.o