busybox: enable noclobber option in install.sh

We support a busybox-menuconfig target so that the BusyBox
configuration can be adjusted as needed.  However, depending
on what other packages are enabled, re-installing BusyBox
symlinks that duplicate "real" apps after the configuration
change can result in bad behaviors:

* At best, the BusyBox applet will be used after the
install, versus the desired "real" app.

* At worst, the built rootfs can become unbootable.

The BusyBox install.sh has some capability to avoid this issue
by means of a --noclobber option.  By default, this option is
disabled.  When enabled, the install.sh will not overwrite a
target file with a symlink or hardlink, be it an actual file
or a previously installed BusyBox link.

The install.sh's argument processing is somewhat broken, so this
patch simply changes the default value of the noclobber option
to on, rather than add --noclobber to the install.sh invocation.

Signed-off-by: Danomi Manchego <danomimanchego123@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>