Error uploading to ATtiny
It's likely that a compiler's file name is missing from a command line the IDE is trying to execute.  To find out more about what the command looks like (which will let you figure out what name is missing), turn on verbose output during compilation.  That is, click File, Preferences in the IDE, then click the box in front of “compilation”.  After you find out what's missing, you can then turn off verbose output and fix platform.txt, which is the file the IDE looks at to find out what commands to execute when compiling sketches.

To get rid of the warnings about undefined build.board values, add lines like the following to the boards.txt file in the tiny/avr/ directory (or whereever you installed it).  (Your 1.6 IDE wants build.board definitions, which IIRC weren't needed by 1.5 and earlier IDEs.)

attiny85-20.build.board=ATTINY_ATTINY85-20


Each of the lines you add will contain the Auto-set value, prefaced by a string made up of a board-type and .build.board= concatenated together.  Add each line just before or after other .build.-containing lines.