base-files: automatically handle paths and symlinks for RAMFS_COPY_BIN

Depending on busybox applet selection, paths of basic utiilties may differ,
and may not work as symlinks to busybox. Simply using whatever binary is
found in PATH and detecting symlinks automatically is more robust and
easier to maintain.

The list of binaries is also slightly cleaned up and duplicates are
removed.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>