mediatek: use U-Boot FAT environment support for Banana Pi R2

Instead of building in a default environment which loads our environment
from the FAT partition....  just ask U-Boot to do it.

Submitted upstream at
https://patchwork.ozlabs.org/project/uboot/list/?series=184688

Signed-off-by: David Woodhouse <dwmw2@infradead.org>

package/boot/uboot-mediatek/patches/005-update-bpir2-defconfig.patch

@@ -2,13 +2,16 @@ diff --git a/configs/mt7623n_bpir2_defconfig b/configs/mt7623n_bpir2_defconfig
 index 6b9fbd7e22..fb2a004803 100644
 --- a/configs/mt7623n_bpir2_defconfig
 +++ b/configs/mt7623n_bpir2_defconfig
-@@ -52,3 +52,9 @@ CONFIG_TIMER=y
+@@ -52,3 +52,12 @@ CONFIG_TIMER=y
  CONFIG_WDT_MTK=y
  CONFIG_LZMA=y
  # CONFIG_EFI_LOADER is not set
 +CONFIG_CMD_BOOTZ=y
 +CONFIG_OF_LIBFDT_OVERLAY=y
 +#enables savenenv-command
-+CONFIG_ENV_IS_IN_MMC=y
++CONFIG_ENV_IS_IN_FAT=y
++CONFIG_ENV_FAT_INTERFACE="mmc"
++CONFIG_ENV_FAT_DEVICE_AND_PART=":2"
++CONFIG_ENV_FAT_FILE="uboot.env"
 +CONFIG_CMD_ASKENV=y
 +CONFIG_ENV_SIZE=0x2000