image_types.bbclass: Set memory usage limit and CPU threads for xz

image_types.bbclass: Set memory usage limit and CPU threads for xz

when building with opkg backend and huge packages e.g. chromium/llvm all
going in parallel, memory pressure causes xz to catapult with

do_package_write_ipk: Failed to create package, opkg-build failed with: xz: (stdin): Cannot allocate memory

since there are many tasks going on in parallel, xz adds to memory pressure
and it wants it all, put an upper limit for memory xz can use

We add a variable XZ_MAXRAM with 30% of RAM limit and can be customized
if builders have more memory one can set it like

XZ_DEFAULTS = "-M 0 -T 0"

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>