Conditionally generate META/boot_filesystem_config.txt.

if BOARD_BUILD_SYSTEM_ROOT_IMAGE != true:  # case A
  - BOOT/RAMDISK corresponds to the / under normal boot, with matching
    fs_config in META/boot_filesystem_config.txt.
  - RECOVERY/RAMDISK corresponds to the / under recovery, with fs_config
    in META/recovery_filesystem_config.txt.
else:
  if BOARD_USES_RECOVERY_AS_BOOT == true:  # case B
    - ROOT/ corresponds to the / under normal boot, with fs_config in
      META/root_filesystem_config.txt.
    - BOOT/RAMDISK corresponds to the / under recovery, with fs_config
      in META/boot_filesystem_config.txt.
  else:  # case C
    - ROOT/ corresponds to the / under normal boot, with fs_config in
      META/root_filesystem_config.txt.
    - RECOVERY/RAMDISK corresponds to the / under recovery, with fs_config
      in META/recovery_filesystem_config.txt.
    - BOOT/RAMDISK doesn't exist.

This CL fixes case C, where we shouldn't try to generate
'META/boot_filesystem_config.txt' for BOOT/RAMDISK. It wouldn't be fatal
without this fix, but would wrongly scan the current directory and
include a large fs_config output into target-files.zip.

Bug: 72731506
Test: `lunch aosp_bullhead-userdebug` and `m dist`. (case A)
Test: `lunch aosp_marlin-userdebug` and `m dist`. (case B)
Test: Define 'BOARD_BUILD_SYSTEM_ROOT_IMAGE := true' for angler. `m
      dist` and check the generated target-files.zip. (case C)
Change-Id: I5582ce8cca464d535af0718be0fd8e65791bd6c2
Merged-In: I5582ce8cca464d535af0718be0fd8e65791bd6c2
(cherry picked from commit da01b211)