busybox: Specify '-nostdlib' when linking to a .o file.

Ensure that linking to the built-in.a file uses the 'nostlib'
option.  Submitted to busybox mailing list here:

    http://lists.busybox.net/pipermail/busybox/2014-March/080730.html

This has been accepted into upstream busybox so it will not be needed
for future versions.

Signed-off-by: Drew Moseley <drew_moseley@mentor.com>