Enable system partition compression

* Prerequisites: external/f2fs-tools: sload compression support
* Must work with corresponding changes in system/extras repository
* If Board config does not change, it falls back to old behavior,
  i.e. no compression for the system partition
* Kernel f2fs compression support is a prerequisite if the Board config
  enables the compression (see below)
* Necessary board config change
 (e.g. device/<vendor>/<product>/BoardConfig-common.mk)

BOARD_SYSTEMIMAGE_FILE_SYSTEM_TYPE := f2fs
BOARD_SYSTEMIMAGE_PARTITION_RESERVED_SIZE := 200000000
BOARD_SYSTEMIMAGE_FILE_SYSTEM_COMPRESS := true
BOARD_SYSTEMIMAGE_F2FS_SLOAD_COMPRESS_FLAGS := <sload compress sub-options>

Setting BOARD_SYSTEMIMAGE_FILE_SYSTEM_COMPRESS to true enables both the
compression support when the initial empty file system be made (mkfs.f2fs)
and the compression flag (-c) when the system image files be side-loaded
by sload.

Sload compress sub-options (i.e. options other than -c) will be provided
by BOARD_SYSTEMIMAGE_F2FS_SLOAD_COMPRESS_FLAGS.   If it is not given, or
is empty, the default sub-options will be used

Please refer to the sload.f2fs manual page.

Setting BOARD_SYSTEMIMAGE_FILE_SYSTEM_TYPE to f2fs is trivially necessary.

* File system table (fstab), notably the file 'fstab.hardware', should
  also changed:
  - The file type must be changed to f2fs
  - Perhaps also other f2fs-specific options

Bug: 170918499
Test: Pixel4a userdebug build (from build id 6918751)
Signed-off-by: Robin Hsu <robinhsu@google.com>
Change-Id: Id9d67b5cb35dc806e06ff1320e89114abc996a28