ccache: set COMPILERCHECK to 'none'

This allows ccache to re-use its cache contents even if the compiler
binary mtime has changed. It is the simplest approach to solve this
problem, and it works for the internal, external and crosstool-ng
toolchain backends.

Of course, it leaves the user responsible for invalidating the cache
when necessary, but there doesn't seem to be a real good solution that
allows both to: 1/ keep the cache contents accross builds and re-use
it and 2/ invalidate the cache automatically when the compiler chances
in an incompatible way.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>