zstd: add libzstd support

zstd: add libzstd support

Add patch to split libzstd install target into pc, static, shared and
includes target. Call only the needed ones for the buildroot
staging/target install steps (respect the static/shared
configuration).

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
[Thomas: as suggested by Yann E. Morin, refactor the build/install
commands to use only one invocation, with intermediate ZSTD_BUILD_LIBS
and ZSTD_INSTALL_LIBS variables, defined depending on whether
static/shared/static+shared is selected.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>