Allow both 4.7 and 4.8 toolchains to exist in parallel, and only update the one you're using if IT is over a day old.

ps14: added usegcc47 and usegcc48 functions to envsetup.
they do what it looks like they would.
their setting persists between shells, so you will be stuck using gcc4.8 after runing usegcc48, UNTIL you run usegcc47.
ps25: added sanity checking for gettop, and fixed permissions issue on temp download location
ps9000 wurx

Change-Id: Ic656a1d71ce9b693d0d8d93f6c60822583907789
Clean up assignment with a useless right-side

Change-Id: I855d553e5c39615bf3e132ec1a02fb9a6936aeae