Bump generic_mips and generic_x86_64 system partition size

This fixes the sdk_google_phone_mips and sdk_google_phone_x86-64
builds

Change-Id: I971d02b07a6aa917d7e0f3750b48e657205aae52
diff --git a/target/board/generic_mips/BoardConfig.mk b/target/board/generic_mips/BoardConfig.mk
index a319ad7..7cef968 100644
--- a/target/board/generic_mips/BoardConfig.mk
+++ b/target/board/generic_mips/BoardConfig.mk
@@ -50,7 +50,7 @@
 USE_OPENGL_RENDERER := true
 
 TARGET_USERIMAGES_USE_EXT4 := true
-BOARD_SYSTEMIMAGE_PARTITION_SIZE := 786432000
+BOARD_SYSTEMIMAGE_PARTITION_SIZE := 838860800
 BOARD_USERDATAIMAGE_PARTITION_SIZE := 576716800
 BOARD_CACHEIMAGE_PARTITION_SIZE := 69206016
 BOARD_CACHEIMAGE_FILE_SYSTEM_TYPE := ext4