Avoid toolchain download when it is preinstalled

For configurations using a toolchain that is preinstalled on
the host, <pkg>_SITE and <pkg>_SOURCE variables must be kept
empty to avoid downloading any toolchain package.
The actual implementation has been proposed by Thomas Petazzoni.

Signed-off-by: GONZALEZ Laurent <br2@gezedo.com>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>