emulator: do not enforce 8G system image partition

Only use 8G as default size, when product does not specify
the system image sizes.

Bug: 265440010
Test: build and check sizes

Change-Id: I8da862482600d06eda9b84d0cc5db65f713067b5
diff --git a/target/board/BoardConfigEmuCommon.mk b/target/board/BoardConfigEmuCommon.mk
index 5acbe7f..7a07d70 100644
--- a/target/board/BoardConfigEmuCommon.mk
+++ b/target/board/BoardConfigEmuCommon.mk
@@ -34,7 +34,7 @@
   BOARD_BUILD_SUPER_IMAGE_BY_DEFAULT := true
 
   # 8G + 8M
-  BOARD_SUPER_PARTITION_SIZE := 8598323200
+  BOARD_SUPER_PARTITION_SIZE ?= 8598323200
   BOARD_SUPER_PARTITION_GROUPS := emulator_dynamic_partitions
 
   ifeq ($(QEMU_USE_SYSTEM_EXT_PARTITIONS),true)
@@ -57,7 +57,7 @@
   endif
 
   # 8G
-  BOARD_EMULATOR_DYNAMIC_PARTITIONS_SIZE := 8589934592
+  BOARD_EMULATOR_DYNAMIC_PARTITIONS_SIZE ?= 8589934592
 
   # in build environment to speed up make -j
   ifeq ($(QEMU_DISABLE_AVB),true)