custom bootimage: Make spelling of variable match the commit in build/

Change-Id: I8c806881f6e78fced4eb181a0a28c3d9ff401612
diff --git a/BoardConfig.mk b/BoardConfig.mk
index 01d8e9e..ba7d4d2 100644
--- a/BoardConfig.mk
+++ b/BoardConfig.mk
@@ -60,4 +60,4 @@
 BOARD_USERDATA_PARTITION_SIZE := 2147483648
 BOARD_FLASH_BLOCK_SIZE := 131072
 
-BOARD_CUSTOM_BOOTIMAGE_MK := device/sony/lt26/custombootimg.mk
+BOARD_CUSTOM_BOOTIMG_MK := device/sony/lt26/custombootimg.mk