mediatek: mt7623: yet another legacy image variable restored

Restore BOOTFS_BLOCKS variable until U7623 gets brushed, so things
hopefully build now.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

target/linux/mediatek/image/mt7623.mk

@@ -113,6 +113,11 @@ endef
 TARGET_DEVICES += bananapi_bpi-r2
 
 
+ifneq ($(CONFIG_MTK_BOOT_PARTSIZE),)
+BOOTFS_BLOCK_SIZE := 1024
+BOOTFS_BLOCKS := $(shell echo $$(($(CONFIG_MTK_BOOT_PARTSIZE)*1024*1024/$(BOOTFS_BLOCK_SIZE))))
+endif
+
 define Build/mtk-mmc-img
 	rm -f $@.boot
 	mkfs.fat -C $@.boot $(BOOTFS_BLOCKS)
@@ -140,6 +145,7 @@ define Build/scatterfile
 		$(subst -scatter.txt,,$(notdir $@)) "$(DEVICE_TITLE)" > $@
 endef
 
+
 # Full eMMC image including U-Boot and partition table
 define Device/unielec_u7623-emmc
   DEVICE_VENDOR := UniElec