RFR: Add clean and images targets to Makefile

[Erik Helin]

Hi,

this patch adds two new "wrapper" targets `images` and `clean` for the Makefile.

Thanks,
Erik

## Testing
- `make images` works
- `make clean` works

----------------

Commits:
 - 9b487b42: Add clean and images targets to Makefile

Changes: https://git.openjdk.java.net/skara/pull/211/files
 Webrev: https://webrevs.openjdk.java.net/skara/211/webrev.00
  Stats: 7 lines in 1 file changed: 6 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/211.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/211/head:pull/211

PR: https://git.openjdk.java.net/skara/pull/211