package/zstd: observe Buildroot optimisation flags

package/zstd: observe Buildroot optimisation flags

Use Buildroot's setting for optimization, zstd's build system
overrides CFLAGS (adds -O3), but MOREFLAGS can override again.
Quick tests show that using -O2 (like buildroot)
is actually a little faster than -O3 on x86_64 Atoms.

Disable the legacy format, these are just needed for
decompressing files created with pre-release version.

Signed-off-by: Norbert Lange <nolange79@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>