package/coreutils: introduce a host variant

More and more packages are now depending on ln --relative, some require
realpath, both of which only got introduced in "recent" versions of
coreutils; older distros had a separate realpath, though, but that is
not in the list of our required dependencies, and was not installed by
default.

So, we introduce a minimal host variant of coreutils to provide those
programs.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>