Reduce the system partition size of generic system images

Some device has small ROM size, so the GSI should be as small as
possible. The patch change the image sizes to be:

- 64 projects (default):    1.25GB (1280MB)
- 32 projects (default):    0.75GB  (768MB)
- ARM32 projects:                    734MB

Bug: 64458958
Test: Build all GSI projects
Change-Id: I7f7a266e48e99b0b4d5c6a84b8187b56e4482c94

target/board/generic_arm_a/BoardConfig.mk

@@ -21,3 +21,7 @@ TARGET_ARCH_VARIANT := armv7-a-neon
 TARGET_CPU_ABI := armeabi-v7a
 TARGET_CPU_ABI2 := armeabi
 TARGET_CPU_VARIANT := generic
+
+# b/64458958
+# Partition size is 734MB for ARM32 (non A/B update)
+BOARD_SYSTEMIMAGE_PARTITION_SIZE := 769654784

target/board/generic_arm_ab/BoardConfig.mk

@@ -25,3 +25,7 @@ TARGET_CPU_VARIANT := generic
 # Enable A/B update
 TARGET_NO_RECOVERY := true
 BOARD_BUILD_SYSTEM_ROOT_IMAGE := true
+
+# b/64458958
+# Partition size is 734MB for ARM32 (A/B update)
+BOARD_SYSTEMIMAGE_PARTITION_SIZE := 769654784

target/board/treble_common.mk

@@ -29,7 +29,6 @@ TARGET_NO_KERNEL := true
 TARGET_USERIMAGES_USE_EXT4 := true
 TARGET_USERIMAGES_SPARSE_EXT_DISABLED := false
 TARGET_USES_MKE2FS := true
-BOARD_SYSTEMIMAGE_PARTITION_SIZE := 1610612736	# 1.5 GB
 
 # Generic AOSP image always requires separate vendor.img
 TARGET_COPY_OUT_VENDOR := vendor

target/board/treble_common_32.mk

@@ -15,3 +15,6 @@
 #
 
 include build/make/target/board/treble_common.mk
+
+# Partition size is default 0.75GB (768MB) for 32 bits projects
+BOARD_SYSTEMIMAGE_PARTITION_SIZE := 805306368

target/board/treble_common_64.mk

@@ -18,3 +18,6 @@ include build/make/target/board/treble_common.mk
 
 # Enable 64-bits binder
 TARGET_USES_64_BIT_BINDER := true
+
+# Partition size is default 1.25GB (1280MB) for 64 bits projects
+BOARD_SYSTEMIMAGE_PARTITION_SIZE := 1342177280