board: mediatek: Use CONFIG_DEFAULT_FDT_FILE for default environment

Rather than hard-coding it to the Banana Pi R2.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>
diff --git a/configs/mt7623n_bpir2_defconfig b/configs/mt7623n_bpir2_defconfig
index 5b753a9..b88067e 100644
--- a/configs/mt7623n_bpir2_defconfig
+++ b/configs/mt7623n_bpir2_defconfig
@@ -13,7 +13,7 @@
 CONFIG_FIT_VERBOSE=y
 CONFIG_BOOTDELAY=3
 CONFIG_SYS_CONSOLE_IS_IN_ENV=y
-CONFIG_DEFAULT_FDT_FILE="mt7623n-bananapi-bpi-r2"
+CONFIG_DEFAULT_FDT_FILE="mt7623n-bananapi-bpi-r2.dtb"
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_SYS_PROMPT="U-Boot> "
 CONFIG_CMD_BOOTMENU=y