Use more sensible names for the external/ctng targets

Instead of using the 'uclibc' target for all toolchain backends,
introduce more sensible target names for the external toolchain and
Crosstool-NG toolchain backend make targets.

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