bitbake.conf: add XZ_INTEGRITY_CHECK and LZMA_COMPRESSION_LEVEL variables

bitbake.conf: add XZ_INTEGRITY_CHECK and LZMA_COMPRESSION_LEVEL variables

Linux kernel has support for XZ Embedded and it has limited support
for integrity checking. Currently only 'crc32' and 'none' are
supported.

This patch allows change it and defaults it to 'crc32' for safeness.

The LZMA_COMPRESSION_LEVEL variable has been add for cpio.lzma image
format since the XZ equivalent variable has already available and in
use. It defaults for best compression ratio.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>