Essentially some more polishing:
- image path extracted from downstream build log;
- extended error/warning regexp a bit so those with
  color grep options get even prettier output.

Notes:
- "1024" a magic number (briefly explained when introduced)
  moved to a sort of variable;
- "100 lines" for tail(1) is a rule-of-thumb taking into account
  typical amount of hasher/mkimage exhaust given GLOBAL_VERBOSE.