package/iputils: add config options to select which binaries to build/install

By default, the iputils build script might build binaries which are
useless for certain applications, like tftpd or ninfod. Those binaries
will add to the target filesystem size unless a post-build script removes
them manually, which is cumbersome and doesn't shorten build times.

In particular, in a certain aarch64 Buildroot project with musl selected
as a C library, this patch allowed to shrink the full iputils package from
219 KiB (if every option is selected) to 63 KiB (with only the ping
binary selected) - a 71.2% relative size decrease.

Moreover, upstream recently introduced a commit that disabled tftpd from
building by default, like rarpd. In the current state of things, this change
will introduce inconveniences for Buildroot users which prefer to use the
tftpd implementation provided by this package. With this patch, however, that
decision and similar future ones won't be a concern, because they will
have complete control of what binaries are built.

These changes add Kconfig options which let the user select what
binaries are built with ease.

Signed-off-by: Alejandro González <alejandro.gonzalez.correo@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>