buildroot

toolchain: rename external toolchain dir

Rename the external toolchain directory.
When new backends are here, it will be easier to sort them out
if they are all prefixed the same way.

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