busybox: clarify BR2_PACKAGE_BUSYBOX_FULLINSTALL description

Explicitly explain what the (default) install target does, so people who
don't know busybox details understands what the option does.

Notice: Busybox can be configured to create hard links or shell wrappers
instead, but by default symlinks are used.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>