toolchain-external: make ext-tool.mk includable in all cases

The ext-tool.mk logic uses the TOOLCHAIN_EXTERNAL_PREFIX variable
unconditionally, even if the external toolchain is not used. Until now
this wasn't a problem since ext-tool.mk was only included when the
external toolchain backend was selected, but the next patches are
going to include this file unconditionally.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>