builtin: Fix echo -n early termination

The commit 7a784244625d5489c0fc779201c349555dc5f8bc ("[BUILTIN]
Simplify echo command") broke echo -n by making it always terminate
after printing the first argument.

This patch fixes this by only terminating when we have reached
the end of the arguments.

Fixes: 7a784244625d ("[BUILTIN] Simplify echo command")
Reported-by: Luigi Tarenga <luigi.tarenga@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>