Full cache image generation

Port forward from here https://gerrit.paranoidandroid.co/#/c/99/ and here https://gerrit.paranoidandroid.co/#/c/103/

Checks goo for latest gapps package through its json api. If the api doesn't return anything, tries to get the link directly from the html. If it
fails too, it checks the /gapps folder to see if there's a package already downloaded.

Cache size can be changed in BoardConfig.mk with the var BOARD_CACHEIMAGE_SIZE (default is 500M).

Change-Id: I3ae608c12a15f6ede2150567662befa7fce763ca