Add support for the XZ compression scheme in the ZBOOT decompression
stub, in order to support it we need to:
- select the "xzkern" compression tool to compress the vmlinux.bin
payload
- link with ashldi3.o for xz_dec_run() to work
- memcpy() is also required for decompress_unxz.c so we share the
implementation between GZIP and XZ