buildroot

toolchain-external: reduce nesting in copy_toolchain_sysroot

As discussed with Thomas Petazzoni, we can reduce the nesting level by early
returning on an invalid iteration.

I did not move the 'else' case (the common case) outside the if-else because
it would make the code less symmetrical and IMO makes it _less_ clear.

Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>