Bump sdk arm system image size to 750M

To fix broken build of arm system images.

Change-Id: I960dbb2a5a895557499fcf38655cd8907e768ef9
(cherry picked from commit 9f42be14b8864c123d3df29ade44e3a92096679e)
diff --git a/target/board/generic/BoardConfig.mk b/target/board/generic/BoardConfig.mk
index 393492b..fb6c920 100644
--- a/target/board/generic/BoardConfig.mk
+++ b/target/board/generic/BoardConfig.mk
@@ -68,7 +68,7 @@
 VSYNC_EVENT_PHASE_OFFSET_NS := 0
 
 TARGET_USERIMAGES_USE_EXT4 := true
-BOARD_SYSTEMIMAGE_PARTITION_SIZE := 576716800
+BOARD_SYSTEMIMAGE_PARTITION_SIZE := 786432000
 BOARD_USERDATAIMAGE_PARTITION_SIZE := 576716800
 BOARD_CACHEIMAGE_PARTITION_SIZE := 69206016
 BOARD_CACHEIMAGE_FILE_SYSTEM_TYPE := ext4