zstd: new package

Add package to provide Zstandard compression tools
(see https://facebook.github.io/zstd

)

Minimal config snippet for utils/test-pkg is as follows:

BR2_PACKAGE_ZSTD=y

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
[Thomas:
 - use "config" instead of "menuconfig" in Config.in
 - add missing final newline in Config.in
 - pass DESTDIR=$(TARGET_DIR) only at install time
 - wrap too long lines in the .mk file
 - remove useless empty newline at end of .hash file.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>