external toolchain: slightly optimize the copy of the toolchain sysroot

The sysroot of an ARM CodeSourcery toolchain takes about 1.4 GB of
space, but 1.1+ GB of this space consists in locale-related
information which Buildroot doesn't use. By skipping the copy of those
unused files, we save quite a bit of time while importing the
toolchain sysroot.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>