manta: increase system partition size

MANTALC02 has a larger system partition (1.3GB), increase the
system image size to match Prime for now (654MB).

Change-Id: Iaf4e25c7ac5493e03ffc13997ce80c779031afcd
diff --git a/BoardConfig.mk b/BoardConfig.mk
index 80c876a..700b248 100644
--- a/BoardConfig.mk
+++ b/BoardConfig.mk
@@ -43,7 +43,7 @@
 USE_OPENGL_RENDERER := true
 
 TARGET_USERIMAGES_USE_EXT4 := true
-BOARD_SYSTEMIMAGE_PARTITION_SIZE := 268435456
+BOARD_SYSTEMIMAGE_PARTITION_SIZE := 685768704
 BOARD_USERDATAIMAGE_PARTITION_SIZE := 536870912
 BOARD_CACHEIMAGE_PARTITION_SIZE := 268435456
 BOARD_CACHEIMAGE_FILE_SYSTEM_TYPE := ext4