Moving debug ramdisk resources under / of the ramdisk

Currently the debug ramdisk resources might be under / or
/first_stage_ramdisk of the ramdisk, and is determined by
some BOARD variables, e.g., BOARD_USES_RECOVERY_AS_BOOT,
BOARD_MOVE_RECOVERY_RESOURCES_TO_VENDOR_BOOT, etc.

To make a generic boot-debug.img that can work on both devices,
let's move the debug resources always under / of the ramdisk.

Bug: 186082603
Test: boot a device with boot-debug.img
Test: boot a device with vendor_boot-debug.img
Change-Id: I55dc8ff322f6b97e2d6dc1a4ee5935e863f2f835