Avoid signing debuggable boot-debug.img

The boot-debug.img should NOT be release signed and can only be used
if the device is unlocked. Adding a check to prevent the tool from
signing this debuggable boot.img.

See the following for more details about boot-debug.img:

  https://android-review.googlesource.com/c/platform/build/+/947857

Bug: 126493225
Test: put a file /force_debuggable into boot.img, checks the following
      command fails:
        ./build/tools/releasetools/sign_target_files_apks \
        out/dist/*-target_files-*.zip signed-target_files.zip
Change-Id: Ia5232949cb9582d2b4eaa171d9e9f3fe7317d418