toolchain/crosstool-ng: allow pre-downloading the sources

On "make source", just download the required sources, and do not
build the entire toolchain. This requires a new patch against
crosstool-NG (got from upstream).

The "uclibc-source" rule requires an installed and configured
crosstool-NG, so it has to be after the variables declaration.
For consistency, move down the main rule "uclibc".

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>