lz4: use CFLAGS from bitbake

Currently lz4 uses it's own defaults which include O3 optimization.
Switch from O3 to bitbake default O2 reduces binary package size
from 467056 to 331888 bytes. Enables also building with Os if needed.

Signed-off-by: Mikko Rapeli <mikko.rapeli@bmw.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>