init: loading debug resources before chroot into first_stage_ramdisk

Currently the debug resources might under /first_stage_ramdisk/*
of the ramdisk, if there is androidboot.force_normal_boot=1 in the
kernel cmdline to request init chroot into /first_stage_ramdisk dir.

To make a generic boot-debug.img works on devices with and without
this chroot, moving the debug resources to the root of the ramdisk.
And copy them for later use before the chroot.

Bug: 186082603
Test: boot a device with boot-debug.img
Test: boot a device with vendor_boot-debug.img
Change-Id: I052a92b2d26c7fdf749991fc55015ff68743efc2