make building recovery and boot images optional

If the source target-files zip omits files needed to build the
recovery and/or boot images, leave them out instead of dying with an
error.  This lets build like "generic-userdebug" work.
2 files changed
tree: 9fe9e78e02a7276d972942eba70c073ae78722d0
  1. core/
  2. libs/
  3. target/
  4. tools/
  5. buildspec.mk.default
  6. cleanspec.mk
  7. envsetup.sh