populate_sdk: Switch from bzip2 to xz

populate_sdk: Switch from bzip2 to xz

xz gives *much* faster decompression times for the SDK which in itself
is a good reason to use it. It also gives better compression.

One downside is its slower but we care about the end user case
first, build performance secondary.

It also assumes the SDK user has a tar capable of understanding a xz
compressed file but that should be common enough now.

(From OE-Core rev: 193086137b899a0a4e774ffc337ed0da9947fd4f)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>