Adding boot-debug.img and ramdisk-debug.img

The two new debugging images adds additional files based on
boot.img and ramdisk.img/ramdisk-recovery.img, respectively.

File /force_debuggable is to trigger special logic in /init to load an
userdebug version of sepolicy and an additional property file from this
ramdisk to allow adb root, if the device is unlocked.

It's intentional to skip signing for boot-debug.img, as it can
only be used if the device is unlocked, where verification error
is allowed.

Those debugging images allows adb root on user build
system.img, vendor.img, product.img, etc. This can facilitate more
automated testings on user builds and is helpful to narrow down the
delta between what's being tested v.s. what's being shipped.

Bug: 126493225
Test: `make dist`, checks both boot-debug.img and ramdisk-debug.img
      are in $OUT/ and out/dist.
Test: `make dist`, checks installed-files-ramdisk-debug.{json,txt} are
       in out/dist.
Test: `system/core/mkbootimg/unpack_bootimg.py --boot_img $OUT/boot-debug.img`,
      checks the extracted out/ramdisk is as expected
Test: Run `gunzip -c ramdisk | cpio -idm` for the ramdisk extracted from
      $OUT/boot-debug.img and $OUT/boot.img, respectively.
      Then compare the root dirs of both, e.g.,
      `diff -rq --no-dereference ./ramdisk ./ramdisk-debug`
Test: `make ramdisk_debug-nodeps` and `make bootimage_debug-nodeps`

Change-Id: I30137c3caef91805d9143d404e5e4d06c0fccc30