scripts/extract-ikconfig: add xz compression support

Add support for kernels compressed with xz to the extract-ikconfig script.
Signed-off-by: Dick Streefland <dick@streefland.net>
Acked-by: WANG Cong <xiyou.wangcong@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>

scripts/extract-ikconfig

@@ -57,6 +57,7 @@ dump_config "$img"
 
 # That didn't work, so retry after decompression.
 try_decompress '\037\213\010' xy    gunzip
+try_decompress '\3757zXZ\000' abcde unxz
 try_decompress 'BZh'          xy    bunzip2
 try_decompress '\135\0\0\0'   xxx   unlzma
 try_decompress '\211\114\132' xy    'lzop -d'