GKI: put debugging resources under fisrt_stage_ramdisk subdir.

For consistency with android-11 GKI boot-debug.img, we should
put debug resources under the 'fisrt_stage_ramdisk' dir. This is
needed for devices with `androidboot.force_normal_boot=1` in the
kernel cmdline, where init will chroot into /fisrt_stage_ramdisk.

For devices without force_normal_boot, they still can use their
vendor_boot-debug.img for debugging purpose.

Bug: 183670217
Test: `make bootimage_debug`, then use unpack_bootimg and
      `lz4 -d -c ramdisk | toybox cpio -i` to unpack the ramdisk
      for inspection.
Change-Id: I0a79440dafd091141a1203a2c2c7be5bc1bfc836