buildroot

zstd: add host libzstd support

For the host only the libzstd library is built (no need for host-lz4
host-xz host-zlib dependencies because they affect only the cmdline
tool).

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
[Thomas: use '-C $(@D)/lib' instead of '-C $(@D) lib' at build time,
to be consistent with what is done at install time, as suggested by
Yann E. Morin.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>