MIPS: ZBOOT: Support LZ4 compression scheme

Add support for the LZ4 compression scheme in the ZBOOT decompression
stub, in order to support it we need to:

- select the "lz4" compression tool to compress the vmlinux.bin
  payload
- memcpy() is also required for decompress_unlz4.c so we share the
  implementation between GZIP, XZ and now LZ4

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: blogic@openwrt.org
Cc: james.hogan@imgtec.com
Patchwork: https://patchwork.linux-mips.org/patch/5829/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>