[Enhancement] Support "-p all" for cache commands

[mrsilver76]

Cache commands (such as --cache validate or --cache purge) require a valid platform to be provided with -p.

It would be useful if use of -p all could be used to apply the appropriate command to all platforms within the cache.

For example: Skyscraper -p all --cache validate

The workaround is that I'll have to write a script that runs Skyscraper -p $PLATFORM --cache validate where $PLATFORM contains the name of each directory located in /home/pi/.skyscraper/cache/.