erlang: enable host-erlang for x86/x86_64 architectures only

And make erlang depends on host-erlang.
This patch aims at avoiding the following autobuilder failures:
http://autobuild.buildroot.net/results/45edf95c0c44c9d553879e0cbb771098d7c63aa1
http://autobuild.buildroot.net/results/a36d00407a371d70b4551a9717ebd6ff852c8bca



Signed-off-by: Johan Oudinet <johan.oudinet@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>