toolchain-external: reduce nesting in copy_toolchain_sysroot

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 <[email protected]>
Signed-off-by: Thomas Petazzoni <[email protected]>