Third-party hardware folders shouldn't need their own platform.txt file. #1157
Description
If you're providing a third-party boards.txt file (to be added to the sketchbook hardware folder), you shouldn't need to provide a platfom.txt file. Instead, the IDE should use the platform.txt file from the package containing the core used by the board.
For example, if I have: attiny45.build.core=arduino:arduino in a boards.txt file, the IDE should use the platform.txt file in the "arduino" folder.
Otherwise, we're going to have a bunch of identical copies of the platform.txt files floating around, which will be a problem if we ever have to make any changes to them.